NOTES: 
  All of the aliens have infra or low light vision, meaning they can function in the dark, and frequently take advantage of that! 
  Besides their plasma weapons, Sectoids, Ethereals, Floaters, Snakemen and Mutons all carry at least 2 alien grenades. 
  The Alien races (except Terrorists) are divided into different classes: Soldier (the basic type); Engineers (who maintain engines and similar functions); Navigators (pilots and navigators who fly the UFOs); medics (who engage in collecting creatures and experimenting with genetics); Leaders (in charge of a craft) and Commanders (in charge of an Alien Base or battleship).
  "Terrorist" creatures are those used to terrorize humans or act as guard units, the are not true alien troops, rather useful allies, slaves, guards or servants. They are normally only found at Terror Mission sites, on Battleships as guards, or in Alien Bases.

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very Rare
ORGANIZATION: Solitary
ACTIVITY CYCLE: Any
INTELLIGENCE: Average (8-10)
TREASURE: Nil
ALIGNMENT: Neutral
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NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: 4
MOVEMENT: FL 9
HIT DICE: 6
THAC0: 15
NO. OF ATTACKS: 1
DAMAGE/ATTACK: 3d6 acid + Save versus Poison or die
SPECIAL ATTACKS: Corrosive (items must save -4), and poison (instantly lethal)
SPECIAL DEFENCES: Psionic resistant +4 saves, thought detection
SPECIAL WEAKNESSES: Only react to living organisms
SIZE: S (3')
MORALE: Champion (16)
XP VALUE: 2,000

Celatids are small, purple blob-like creatures, they are natural hunters and like skulking in shadowy places and attack from above or concealment. They are allies of the Muton race.
COMBAT: They are highly aggressive hunters, they track using their quarries' brain waves, this means that their victims cannot hide from them!! The range of this ability is 30 yards, they can also sense non-living material but less keenly, for instance they have trouble recognizing mechanical constructs, such as tanks, as possible threats.
Their main weapon is a highly corrosive spit, which has a range of 30', it does 3d6 hp but it is also highly poisonous, victims must save versus poison or die instantly! Victims in armour better than Kevlar have until the end of the round to neutralize the poison some way before they are affected. Items (including Heavy Weapon Platforms, HWPs) struck by the venom must save versus acid or be destroyed or damaged, in the case of armour it would lose 1 point of AC value.
They are very resistant to psionics, getting a +4 save bonus.
HABITAT/SOCIETY/ECOLOGY: Celatids seem to be hunting creatures, perhaps from a dark or crowded environment where eyes are of little use. They can fly thanks to a bizarre natural anti-gravity system, and this seems to have been enhanced by cybernetic implants. Celatids seems to have no recognizable internal organs, but it does have a small embryo sack, the embryos are already fertilized and are dormant until the creature wishes to reproduce. However it seems that most Earth climates are not to their liking and prohibit breeding, swamps seem to be their favoured habitat and it would not be pleasant if some of them started breeding there....
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very Rare
ORGANIZATION: Solitary
ACTIVITY CYCLE: Night
INTELLIGENCE: Average (8-10)
TREASURE: Nil
ALIGNMENT: Chaotic Evil
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NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: 0 (4 without dexterity)
MOVEMENT: 24
HIT DICE: 9+3
THAC0: 10
NO. OF ATTACKS: 4
DAMAGE/ATTACK: 2d6+10, + embryo injection
SPECIAL ATTACKS: Embryo injection, strength
SPECIAL DEFENCES: speed, regenerate 1 hp/turn
SPECIAL WEAKNESSES: Fire does +50% more damage, stun attacks do +10% more damage
SIZE: M (7')
MORALE: Fearless (20)
XP VALUE: 9,000

Chryssalids are large, crab-like horrors who can move incredibly swiftly. They are also unbelievably strong and viciously aggressive. Their ability to impregnate victims with embryos, turning them into zombies and later another chryssalid makes them one of the most feared aliens of all. Often they are found with the Snakeman race.
COMBAT: Extremely fast they can run as fast as horse! Typically they use half their movement to rush up to a foe and then still get half their attacks. Physically they are incredibly developed (Dexterity 18 and Strength 22), and their super-fast metabolic rate also lets them regenerate non-fire or acid wounds at 1 hp/turn. However this makes them vulnerable to stun weapons (+10% damage, -2 saves). Their exoskeleton is vary badly effected by fire attacks and they suffer +50% damage (-4 saves).
Chryssalids rarely kill living targets, instead they stop the blow from maiming and inject the victim with a chryssalid embryo, the victim then turns into a mindless zombie. Later the zombie falls apart and the new chryssalid emerges.
  Their extreme strength allows them to destroy XCOM HWPs (Heavy Weapon Platforms) by tearing open armour plates and damaging control circuitry. In such cases a Saving Throw vs. Crushing Blow is needed.
They use sound and heat to hunt by and preffer darkness to operate in.
HABITAT/SOCIETY/ECOLOGY: Although Chryssalids have well developed brains, they only seem to think of breeding and slaughter. They are very dangerous creatures indeed. Normally they travel with Snakemen.
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very Rare
ORGANIZATION: Solitary
ACTIVITY CYCLE: Any
INTELLIGENCE: High (14)
TREASURE: Nil
ALIGNMENT: Neutral
-------------------------------------------------------------------
NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: 0
MOVEMENT: Fl 12
HIT DICE: 12
THAC0: 8
NO. OF ATTACKS: 3
DAMAGE/ATTACK: 11d4
SPECIAL ATTACKS: Nil
SPECIAL DEFENCES: 1/2 damage from explosive and armour piercing attacks, immune to pisonics
SPECIAL WEAKNESSES: Nil
SIZE: L (10' wide)
MORALE: Fearless (20)
XP VALUE: 7,000

Cyberdisks are advanced robots, built and functioning much like a flying saucer, they mount a heavy plasma gun as their main weapon. They are generally built and operated by the Sectoid race.
COMBAT: Flying around using an antigravity system, cyberdisks act as scouts and tanks, and are armed with a built in device equal to a heavy plasma rifle. Their programming is sophisticated and they are good at sneaking around and ambushing folk, then flying into safe cover. Their incredibly thick armour makes them resistant to armour piercing and explosive attacks (taking only half damage from both, +4 saves). Being robotic they are immune to psionics.
One really nasty problem with cyberdisks is that there's a 50% chance they explode when destroyed! The blast does 10d6 damage.
HABITAT/SOCIETY/ECOLOGY: Used by the Sectoids for combat, they have no ecology or society to speak of, but they are intelligent in a non-conscious way.
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)

CLIMATE/TERRAIN: Unknown
FREQUENCY: Rare
ORGANIZATION: Gestalt?
ACTIVITY CYCLE: Any
INTELLIGENCE: Genius (17-18)
TREASURE: varies
ALIGNMENT: Lawful Evil
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NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: 0
MOVEMENT: 12, Fl 12
HIT DICE: 6+3
THAC0: 14
NO. OF ATTACKS: by weapon (3 with plasma gun)
DAMAGE/ATTACK: by weapon type
SPECIAL ATTACKS: Psionics
SPECIAL DEFENCES: Resistant to pisonics (+4 save), fire and stun damage (+2 saves, -25% damage)
SPECIAL WEAKNESSES: Nil
SIZE: M (7'tall)
MORALE: Fanatic (17)
XP VALUE: 3,000

Ethereals are the leaders of the alien races, but they are generally only to be found with others of their own kind, or running an alien base with other races as their servants. Mutons however seem to be dependant on them and are occasionally found to be guided by an ethereal.
COMBAT: Very powerfully psionically, ethereals are still physically dangerous despite their atrophied and seemingly weak bodies. Their strength and flying abilities seem to be due to their psionic powers. Typically they are armed with Heavy Plasma Rifles, see Items for details.
All ethereals can use psionics to attack victims, see Combat for more details on psionics. Each ethereal therefore can make 18 psionic attacks per day, either to cause Fear or Dominate their victim. Naturally they are resistant to psionics getting a +4 to saves.
Fire and stun damage are less effective against them, they get a +2 save and -25% damage bonus.
Other ranks of ethereals exist: medics are armed with small launchers to capture victims; Engineers; Leaders have 8+3 HD THAC0 12; Commanders have 10+3 HD THAC0 9 and their victims have a -4 save against the commander's  psionics!
HABITAT/SOCIETY/ECOLOGY: Rarely seen on Earth these creatures are the leaders of all the aliens races, coordinating and planning their strategies. While their bodies are withered and seemingly useless, their mental powers are incredibly formidable and these seem to keep them alive. It is possible that ethereals have a sort of "gestalt" or linked racial mentality.
Ethereals use robotic Sectopods as their elite guards and tanks, and rely on Mutons as their shock troops.

CLIMATE/TERRAIN: Unknown
FREQUENCY: Common
ORGANIZATION: Military
ACTIVITY CYCLE: Any
INTELLIGENCE: Average (10)
TREASURE: varies
ALIGNMENT: Lawful Evil
-------------------------------------------------------------------
NO. APPEARING: 3-12 (according to ship type)
ARMOUR CLASS: 6
MOVEMENT: Fl 12
HIT DICE: 3+3
THAC0: 16
NO. OF ATTACKS: by weapon (3 with plasma gun)
DAMAGE/ATTACK: by weapon type
SPECIAL ATTACKS: Nil
SPECIAL DEFENCES: Nil
SPECIAL WEAKNESSES: Nil
SIZE: M (6'tall)
MORALE: Elite (14)
XP VALUE: 420

Floaters seem to be one of the most common alien types. They are aggressive, natural hunters. It is obvious they have been genetically and cybernetically modified. Adapted to live in harsh environments, Floaters have had their lower limbs removed and replaced with antigravity units, allowing them to fly.
COMBAT: Floaters have no special abilities beyond their ability to fly and use of weapons, they can use any of the exotic alien weaponry, most often a plasma rifle, see Items for details.
Other ranks of ethereals exist: medics are armed with small launchers to capture victims; Navigators; Engineers; Leaders have 5+3 HD THAC0 15; Commanders have 7+3 HD THAC0 12.
HABITAT/SOCIETY/ECOLOGY: These monsters are one of the primary alien races, and serve as soldiers and general workers. Oddly their heavy modifications means that their life support systems (heart, lungs, digestion) are housed in the sphere at the base of their bodies.
Reapers are frequently encountered with Floaters at Terror Sites, in bases or as guards in their Battleships.

CLIMATE/TERRAIN: Unknown
FREQUENCY: Uncommon
ORGANIZATION: Primitive
ACTIVITY CYCLE: Any
INTELLIGENCE: Low (5)
TREASURE: varies
ALIGNMENT: Lawful Neutral (Evil)
-------------------------------------------------------------------
NO. APPEARING: 3-12 (according to ship type)
ARMOUR CLASS: 3
MOVEMENT: 12
HIT DICE: 12 (96 hp)
THAC0: 9
NO. OF ATTACKS: by weapon (3 with plasma gun), or 2 melee
DAMAGE/ATTACK: by weapon type, 1d4+10 in melee
SPECIAL ATTACKS: High strength
SPECIAL DEFENCES: Armour piercing attacks do only 1/2 damage, regenerate 1hp/hour
SPECIAL WEAKNESSES: -4 save versus Psionics
SIZE: M (7'tall)
MORALE: Fanatic (17)
XP VALUE: 5,000

Mutons are a savage, carnivorous race, how have been heavily modified to make them unbelievably tough. They are the "shock troops" of the alien races. Their green skin is actually organic armour.
COMBAT: Designed purely for combat, Mutons are very good at it! Physically very strong and robust (Strength 22 and Constitution 20), they can take and give a tremendous amount of punishment. They never have less than 8 hp per die (for a standard Muton that gives 96 hp!), and often use their strength to shatter through doors and even walls, they can hurl humans around like rag dolls.
Their organic skin absorbs blows from armour piercing projectiles very effectivley, reducing damage by 1/2. Mutons have acute sense, including infra and low light vision.
Mutons main weakness is their stupidity, they have a very low intellect, and this also makes them vulnerable to psionics, suffering a -4 save penalty.
  Their favoured weapon is the Heavy Plasma Rifle.
Other ranks of ethereals exist: medics are armed with small launchers to capture victims; Navigators; Engineers; Leaders have 14 HD THAC0 9; Commanders have 16 HD THAC0 5.
HABITAT/SOCIETY/ECOLOGY: Apparently the Muton race is heavily dependant on Ethereals, wihout a telepathic link to them eventually their mental system starts to fail and they die! When encountered on very large ships such as Terror Ships, Supply Ships and Battleships, rarely there maybe an Ethereal in charge of them, in such cases the Ethereal never uses psionic attacks as it's too busy controlling the Mutons, but that means the Mutons act intelligently!
Occassionally Celatids are encountered with Mutons, they seem to be allies.

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very Rare
ORGANIZATION: Pack?
ACTIVITY CYCLE: Any
INTELLIGENCE: Semi (4)
TREASURE: Nil
ALIGNMENT: Neutral
-------------------------------------------------------------------
NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: 3
MOVEMENT: 15
HIT DICE: 15 (120 hp)
THAC0: 5
NO. OF ATTACKS: 4
DAMAGE/ATTACK: 4d4+8
SPECIAL ATTACKS: Nil
SPECIAL DEFENCES: Nil
SPECIAL WEAKNESSES: suffers double damage from fire attacks.
SIZE: H (9'tall 15' long)
MORALE: Fearless (20)
XP VALUE: 6,000

Reapers are enormous, furry, carnivorous bipeds. They aren't intelligent and are used as combat units by the Floaters.
COMBAT: Reapers massive bodies can sustain a huge amount of damage, as all of their important survival systems are duplicated (heart, lungs etc), as well as being cybernetically enhanced. For this reason they never have less than full hit points (120 hp).
Utterly fearless and ferocious, they only attack with their fanged jaws or powerful legs. Like organic bulldozers they can smash through walls (their armoured heads help in this).
They do have one weakness though: their fur is very oily and flammable, hence fire attacks do double damage.
HABITAT/SOCIETY/ECOLOGY: Obviously a simple but tough carnivore from some unknown world, they are now the servants of the Floater race.
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)

CLIMATE/TERRAIN: Unknown
FREQUENCY: Common
ORGANIZATION: Hive?
ACTIVITY CYCLE: Any (night)
INTELLIGENCE: Low to Exceptional (5-16)
TREASURE: varies
ALIGNMENT: Lawful Neutral (Evil)
-------------------------------------------------------------------
NO. APPEARING: 3-12 (according to ship type)
ARMOUR CLASS: 8
MOVEMENT: 12
HIT DICE: 3
THAC0: 18
NO. OF ATTACKS: by weapon (3 with plasma gun)
DAMAGE/ATTACK: by weapon type
SPECIAL ATTACKS: Possible psionics
SPECIAL DEFENCES: Nil
SPECIAL WEAKNESSES: Nil
SIZE: S (4-5'tall)
MORALE: Fanatic (17)
XP VALUE: 175

Sectoids seem to be a sort of "cloned race", and are the species most associated with alien abductions and cattle mutilations.
COMBAT: The typical sectoid has little in the way of natural combat skills. They tend to be armed with plasma pistols or plasma rifles and alien grenades. One danger with this race is that they have little sense of "self", ergo they can be suicidal! A favourite tactic is for a sectoid to prime a grenade and rush into attack the enemy at close quarters, when it's killed it drops the grenade and BOOM!
Sectoids have low light vison and can function equally well in day or night time conditions
Other ranks of sectoids exist with increasing intelligence: medics are armed with small launchers; Navigators; Engineers; Leaders have 5 HD THAC0 16 psionics usable 12 times per day XP 650; Commanders have 8 HD THAC0 13 Psionics usable 16 times per day XP 1,400.
HABITAT/SOCIETY/ECOLOGY: Sectoids are form of cloned race, and they are heavily involved with alien abduction and cattle mutilations. This is linked with alien/human hybrid breeding, infiltration and feeding. The sectoid race has lost the ability to reproduce or ingest food naturally.
Occassionally Cyberdisks are encountered with Sectoids, Cyberdisks are their robotic attack servants.

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very rare
ORGANIZATION: Cybernetic
ACTIVITY CYCLE: Any
INTELLIGENCE: Average (10)
TREASURE: Nil
ALIGNMENT: Neutral
-------------------------------------------------------------------
NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: -10
MOVEMENT: 12
HIT DICE: 12 (96 hp)
THAC0: 9
NO. OF ATTACKS: 3 with laser
DAMAGE/ATTACK: 10d4 laser
SPECIAL ATTACKS: Nil
SPECIAL DEFENCES: -25% damage +2 saves vs plasma and high explosive
SPECIAL WEAKNESSES: +50% damage +4 to be hit with lasers
SIZE: H (9'tall 15' long)
MORALE: Fearless (20)
XP VALUE: 8,000

Sectopods are massive robots used and controlled by the Ethereal race. Fortunately they are rarely met for they are formidable foes.
COMBAT: Armed with a heavy laser weapon, and armoured in thick plates, with internal compartmentalization, these devices are very hard to knock out. They are resistant to explosive and plasma weapons, taking -25 % less damage (+2 saves where appropriate).
They do have a major weakness though: laser weapons are very effective against them, especially as lasers damage their sensory systems, thus they take an extra 50% damage (-4 saves where appropriate).
One other vulnerability can be exploited: their under armour isn't as thick so setting off an explosive literally below their feet can be an effective tactic (full damage applies in that case). Also since they don't have arms, if they are toppled over they can't right themselves.
HABITAT/SOCIETY/ECOLOGY: As servants of the Ethereals, Sectopods will only be found with their masters
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very rare
ORGANIZATION: Unknown
ACTIVITY CYCLE: Any
INTELLIGENCE: Animal (1)
TREASURE: Nil
ALIGNMENT: Neutral
-------------------------------------------------------------------
NO. APPEARING: 1-3 (according to ship type)
ARMOUR CLASS: -4
MOVEMENT: 9
HIT DICE: 11
THAC0: 9
NO. OF ATTACKS: 3
DAMAGE/ATTACK: 7d4 heat
SPECIAL ATTACKS:Possible fire hazard
SPECIAL DEFENCES: Immune to fire
SPECIAL WEAKNESSES: +25% damage +2 saves verus high explosive
SIZE: S (4'tall)
MORALE: Elite (14)
XP VALUE: 5,000

Silacoids are small, rock -like creatures. They give off tremendous amounts of heat and use this as a weapon.
COMBAT: Silacoids attack by touching their victims, and burning them. Since they are essentially thermic creatures, fire attacks have no effects on them. A danger with these creatures is that they can start large fires as they move. Also when they are killed they release their molten cores, the heat from which burns everything in a 15' radius for 7d4 hp.
Explosives inflict 25% extra damage (-2 saves) as it shatters their stony bodies.
HABITAT/SOCIETY/ECOLOGY: These creatures actually eat and subsist on rocks. very primitive mentally, they are controlled by cybernetic or telepathic means. They are servants of the Muton race.
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)

CLIMATE/TERRAIN: Unknown
FREQUENCY: Common
ORGANIZATION: Unknown
ACTIVITY CYCLE: Any
INTELLIGENCE: Low (5-7)
TREASURE: varies
ALIGNMENT: Neutral
-------------------------------------------------------------------
NO. APPEARING: 3-12 (according to ship type)
ARMOUR CLASS: 5
MOVEMENT: 9
HIT DICE: 4+3
THAC0: 17
NO. OF ATTACKS: by weapon (3 with plasma gun), 3 melee
DAMAGE/ATTACK: by weapon type, 1d4+4/1d4+4/1d6+4 melee
SPECIAL ATTACKS: Nil
SPECIAL DEFENCES: -25% damage from fire attacks (+2 saves)
SPECIAL WEAKNESSES: Nil
SIZE: L (5' tall 11' long)
MORALE: Fanatic (17)
XP VALUE: 975

Snakemen are a reptilian race, they can often be heard moving before they are seen due to their need to "slither" for movement.
COMBAT: Physically Snakemen are quite tough and robust, having 18/79 Strength typically. They can calw and bite in melee if they have to. Their favoured weapons are plasma rifles and heavy plasma rifles, and of course the ubiquitous alien grenade.
Coming from a harsh world, Snakemen are resistant to fire, taking -25% damage and getting a +2 saving throw bonus.
Other ranks of Snakemen exist with increasing intelligence (up to Int 14): medics are armed with small launchers; Navigators; Engineers; Leaders have 6+3 HD THAC0 15  XP 1,400; Commanders have 8+3 HD THAC0 13 XP 2,000.
HABITAT/SOCIETY/ECOLOGY: The Snakemens' vital organs are actually in their muscular "foot", it also acts as a hydraulic system as that is their primary method of muscular power!
Occasionally Snakemen are encountered with Chryssalids, the two species seem to get on well, peculiarly.

CLIMATE/TERRAIN: Unknown
FREQUENCY: Very rare
ORGANIZATION: Unknown
ACTIVITY CYCLE: Any
INTELLIGENCE: Animal (1)
TREASURE: Nil
ALIGNMENT: Neutral
-------------------------------------------------------------------
NO. APPEARING: varies
ARMOUR CLASS: 10
MOVEMENT: 9
HIT DICE: 8
THAC0: 14
NO. OF ATTACKS: 2
DAMAGE/ATTACK: 1d4+6
SPECIAL ATTACKS: Chryssalid impregnation, Chryssalid birth, strength
SPECIAL DEFENCES: Immune to stun attacks, 1/2 damage vs armour piercing, -25% from explosive, laser and plasma, +4 save vs. psionics
SPECIAL WEAKNESSES: Nil
SIZE: M  (6'tall)
MORALE: Fearless  20)
XP VALUE: 2,000

Zombies are the result of a Chryssalid atatcking a human, the victim is impreganated with a Chryssalid embryo and wanders around trying impregnate more folk with the Chryssalid spawn before the Chryssalid inside bursts free!
COMBAT: Zombies attack by clawing their victims, since they are slo slow, albiet strong (Str 18/00), they have a poor THAC0. Victims struck are impregnated by a chryssalid and themselves turn into zombies!
When killed, or after half an hour since impregnation, the zombie bursts open, releasing a new chryssalid.
Thanks to their peculiar physiology, zombies are immune to stun attacks, and take 1/2 damage from armour piercing weapons, -25% from explosive and laser attacks and their damaged brains give them a +4 save versus psionics.
HABITAT/SOCIETY/ECOLOGY: These poor, disgusting wretches were once fellow humans, now the best that can be done is to destroy them before they spread the chryssalids' foul spawn.
These monsters are generally found only at Terror Sites (see missions) and Alien Bases (see goals and plots)