There are a number of Alien Life Forms to be encountered during the First Alien War, each with a unique set of strengths, weaknesses and abilities. Know your enemy; your soldiers' lives depend on it.

All weapons carried by the aliens explode when they are killed. Captured weapons are indistinguishable from built weapons of the same type.

Races

Sectoid

Sectoids are a basic, weak unit with an average ranged attack. They are known to be extremely aggressive in numbers and timid / cowardly (i.e. start camping/overwatch) if their number is few. They are capable of some psionics. Killing a unit that is performing Mind Merge will also kill the unit receiving the merge. This makes sectoids in numbers not as dangerous as half of them will consistently merge minds instead of shooting your agents, and giving you a double kill if you take out the mind merger (not the one being mind merged)

Mind Merge: the merged ally receives +1 hit points, increased accuracy, & increased critical hit chance.

Killing a sectoid will leave behind 2x weapon fragments.

Thin Man

Thin Men are weak, highly mobile units with an average ranged attack. They use a light plasma rifle, making them more accurate than Sectoids, but they don't pack as much firepower as other enemies.

Poison Spit: Causes damage each turn for up to 3 turns to any unit poisoned by the cloud of spit. Being poisoned reduces a unit's accuracy by 20%.
Thin Men explode into a cloud of poison upon death. Moving into this cloud has the same effect as being poisoned by their spit attack.
Leap: Allows the Thin Man to jump vertically up onto high ground (e.g., roof tops).
Note: Units carrying a medikit or titan armor are immune to the Thin Man's poison.
Trivia: The Thin Man's face is a caricature of Sid Meier, boss of Firaxis. Sid's face has a habit of popping up in his games.

Gray Market description: "The Thin Man cadaver begins to severely decomposed severely decompose within hours after death, giving off an overwhelming stench which may relate to toxins stored within its body"

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Thin Man Plasma Light Rifle	4	10	125	27	200	NA	Large	Suppression 20%

Floater

A fast and aggressive flying unit with a light plasma rifle. They prefer to use their mobility to outflank your squad, and their high mobility grants them an innate defensive bonus and negate exposed critical bouns. They can be especially dangerous to Snipers.

Launch: The Floater can launch to any point on the map (ends its turn).
Floaters are always considered to have at least Light cover even in the open or when flying, unless in proper cover and Flanked. This bonus does not stack with existing cover bonuses - they only recieve the best cover.

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Floater Plasma Light Rifle	6	10	125	27	200	NA	Large	Suppression 20%

Heavy Floater

A tougher version of the regular Floater with considerably more hit points than a Floater, and armed with a Heavy Plasma. They also carry an Alien Grenade.

Evasion: Enemies targeting this unit when it is airborn suffer a -20 Aim penalty.
Launch: Move anywhere on the battlefield in a single turn. Launch cannot be used indoors.
Bombard: Throw or launch grenades over exceptionally long distances.

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Floater Heavy Plasma	10	10	125	27	200	27	Large	Suppression 35%

Muton

Mutons are very strong units and carry a respectably powerful Plasma Rifle, though they are susceptible to psionics. They are alien's version of x-com's agents and will aggressively use tactics such as flanking /overwatch/suppression against your soldiers. Wounding a Muton without killing it can intimidate your soldiers and potentially cause panic.

Suppression: Can fire a special shot that grants reaction fire at a single target. The target also suffers a -30 Aim penalty.
Blood Call: Unleash a bestial roar that inspires nearby allies of the same species to greater speed, focus, and resolve for 2 turns. 4 turn cooldown.
Intimidate: Reacts unpredictably when wounded, provoking panic in enemies.
Mutons carry Alien Grenades, allowing them to injure multiple troops at once and destroy cover.
They carry light rifles on first month they appear, and rifles after that month.

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Muton Plasma Light Rifle	6	10	125	27	200	NA	Large	Suppression 20%
Muton Plasma Rifle	8	10	125	27	200	NA	Large	Suppression 20%

Muton Elite

An even tougher version of the regular Muton, armed with a powerful Heavy Plasma. Their powerful weapons and good aim and the fact that they appear in groups means they can severely punish you each turn they're allowed to fire, while their ridiculous innate defense and ability to use cover makes them difficult to beat in a conventional firefight. Their major weakness is to psionic attack.

Suppression: Can fire a special shot that grants reaction fire at a single target. The target also suffers a -30 Aim penalty.
Bombard: Throw or launch grenades over exceptionally long distances.
Enemy Defense: +20%/+30% (normal / classic)

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Muton Heavy Plasma	10	10	125	27	200	27	Large	Suppression 35%

Berserker

Berserkers are extremely strong and very tough melee units, susceptible to their own bloodlust; they have a habit of chasing after whichever unit damaged them last (uncontrollable in multiplayer?). Although aggressive in nature, they can exhibit some tactical decision to fall back and regroup with other mutons when not wounded (once they are wounded they go berserk and charge toward the nearest xcom agent). They are also quite resilient to psionic attack. A suppressed berserker will tend not to take any action.

Bull Rush: Charges through cover and causes damage to any units within a small radius.
Blood Lust: Allows the Berserker to charge an enemy that wounds it.
Hardened: Hardened units receive extra protection against critical hits (-60% chance to crit)
Intimidate: Reacts unpredictably when wounded, provoking panic in enemies

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Muton Blade	10	33	125	2	NA	Small	No reaction shot

Outsider

Outsiders are relatively fragile alien units that are only encountered on UFO missions. They are the early commander type units that will remain in their chamber on the UFO until discovered. Their purpose seems to be to defend that position, as they will not go far beyond it. They will be replaced by sectoid commander / ethereal after you assaulted alien base or shot down the overseer UFO.

capturing one is required to progress the main plot.

Weapons Used
Weapon	Damage	Critical Chance	Terrain Damage	Range	Reaction Angle	Reaction Range	Size	Abilities
Outsider Plasma Light Rifle	4	10	0	27	200	NA	Large	Suppression 30%

Chryssalid

Chryssalids are fast melee units. Their Implant attack is particularly dangerous. Their corpses can be used to make Chitin Vests, an equippable item that makes your soldiers tougher and more resilient to melee damage. Since melee attacks ignore cover, Chryssalids are best fought by abandoning cover completely, falling back into the open, and concentrating your fire. Note: Chrssalids are where the changes in the action system are most notable. While not fundamentally different from their counterpart in the original, so long as you are far enough away to force them to dash they are significantly less dangerous, often running right up to your soldiers and ending their turn there.

Stun Immune: This unit cannot be stunned with the Arc Thrower; taking a live specimen is not possible.
Leap: Allows vertical leaps onto elevated surfaces during movement.
Poisonous Claws: "Poison enemies wounded by a melee attack."
Implant: "Victims are implanted with a Chryssalid egg if they are killed by the attack. (not from poison caused by Poisonous Claws." - A successful implant converts the victim into a zombie that eventually hatches into another Chryssalid a couple turns later. Killing the zombie seemingly prevents this transformation from occurring.
Hardened: Hardened units receive extra protection against critical hits (-60% crit chance).

Cyberdisc

Cyberdisc - open assault form

Cyberdisc - closed disc form

Cyberdiscs are strong, flying robotic units with a powerful plasma barrage attack that frequently destroys cover even if it fails to hit the target. They are an extremely aggressive unit similar to floaters. Unlike floaters, they never back down regardless of health / remaining aliens. When in their 'closed' form, they take less damage and are highly resistant to Critical Hits. They are always accompanied by a pair of Drones, which can repair them during combat. They are immune to most psionic attacks.

Can throw an Alien Grenade to attack multiple soldiers at once.
Cyberdiscs automatically 'close-up' into their disc form after being attacked. In their closed form, they are almost immune to Critical Hits.
Death Blossom: An area of effect attack dealing high damage to all units within a few spaces in all directions.
Cyberdiscs explode upon death, dealing damage to all units within a fairly small radius.

Drone

Drones are weak flying robotic units with a weak ranged attack. Their purpose is to repair accompanying Cyberdiscs and Sectopods during combat.

With Foundry Upgrades, the Arc Thrower can be used to 'hack' Drones and seize control of them for the duration of a mission.
Captured Drones can not repair friendly SHIV. However, they can self-detonate, dealing damage similar to frag grenade.

Ethereal

The ultimate alien. High health, highest innate defense and powerful psionic attacks. They do not take cover despite being humanoid (however is hardened, -60% crit). They do not carry physical weapons.

Psi Lance: Offensive Psi ability that deals more damage if the target fails a will challenge.
Mindfray, offensive psionic ability that causes damage and lowers will/mobility/aim.
Can Mind Control.
Psi Drain: Drains up to 5 hitpoints worth of health from a friendly organic target. Cannot kill target(Can be used on mind-linked targets such as mind controlled squaddie)
Rift, a powerful AoE attack that inflicts more damage if they fail a will challenge. It has the potential to do from 4 damage (if resisted) to 20+ (SHIV, since they have no will to resist.) Rift will persist on the map for one turn and damage anything that enters.
Chance to reflect physical projectiles back at the attacker.
High Will; resistant to psionic attacks.
Ethereals explode upon death, dealing damage to all units within a fairly small radius.

Sectoid Commander

The Sectoid Commander is a slightly stronger Sectoid with more psionic capabilities. They have much more health than normal Sectoids, but are otherwise unremarkable aside from their Mind Control ability.

Mind Fray: Causes the target to lose grip on reality, inflicting penalties to Aim, Will, and mobility and doing 5 base damage. Lasts 2 turns, 1 turn cooldown.
Mind Control: Very difficult psi technique that, if successful, grants control of the target for 3 turns.
Greater Mind Merge: Merge Minds with all lesser allies of the same species nearby, granting them +25% critical chance and +1 health.
Enemy Defense: -20% Hit

Carries Plasma Pistol & Alien Grenade.
Will use psi attacks against targets with a low Hit chance. If trying to capture one, sometimes its best to leave a soldier out of cover and eat a plasma pistol shot instead of taking a psi attack.

Sectopod

An extremely strong robotic unit that can use both its turn moves for attacking, but only at different targets. Frequently escorted by Drones. They are not as aggressive as cyberdiscs but significantly more dangerous. They don't move around a lot and will prefer to bombard your agent from far away. They are immune to most psionic attacks. Make as much use of Smoke Grenades, Heavy Cover, and Psionic Fields as you can if you do end your turn within their sights.

Cannon Fire: Beam attack that causes high damage and grants free Overwatch. If the Sectopod does not move, Cannon Fire can be used twice (against different enemies).
Cluster Bomb: Mark a large area, and after one turn, saturate it with a barrage of explosive minibombs. - Uses both of the Sectopod's actions for that turn. It enters a special deployed state and aims at the target location; on the following turn, it bombards the target area with many small explosives. Do note this ability has no range limit and can be fired if another alien spot you (like sniper's squad sight, except it doesn't need a line of sight). While channeling this ability, the sectopod will not take another other action including its plasma auto-overwatch turrets.
Hardened: Hardened units receive extra protection against critical hits (-60% crit chance)

Sectopod's chest cannon and auto-overwatch plasma turret deal area damage if hits someone, including destroying cover. This makes having two soldiers very close a major mistake, because not only will the Sectopod damage both soldiers if it hits one, it can destroy their cover and then fire on the other soldier with no cover bonus.
Seems to have a longer sight range than most units.
Note: Due to its superior firepower, use of suppression may be necessary to render its attacks far less dangerous, but practically guaranteeing a double attack. Do not stack units too close together or the Sectopod may choose to target them with the Cluster Bomb attack. Using Plasma / Heavy weapons or Mind-controlled cannon-fodder against this unit is also recommended.
The Heavy's HEAT Ammo promotion applies to all weapons, and can allow them to inflict vast amounts of damage to Sectopods; combined with Bullet Swarm, a pre-positioned Heavy with a Heavy Plasma can easily kill or cripple a Sectopod in one turn, or even destroy two that have already taken significant damage. HEAT ammo also affects rockets. A non-crit with a Heavy's rocket can do 16 points of damage.
The Heavy's Shredder Rocket promotion can also be used to make the Sectopod take additional damage from all other attacks.
Snipers can use their Headshot or Disabling Shot powers to cripple or inflict serious damage to a Sectopod, especially if they have the Double Tap promotion.
If you see Cluster Bomb being prepared, RUN.

Uber Ethereal

Same as the Ethereal, but with larger stats and a higher projectile reflection chance.

Alien Stats Table

Name	HP (Easy/Normal/Classic/Impossible)	Aim	Defense	Will	Movement	Notes
Sectoid	3/3/3/4	65	0	10	12	Target of a mind merge gains one extra health, healed effectively. Increases critical hit chance and will too.
Outsider	3/3/5/5	70	0	20	12	Receives an additional +10 Aim and +5 Movement from bonuses on impossible. Tied for highest accuracy in the game.
Thin man	3/3/4/6	65	0	15	15	Deals 2 less damage on normal/easy
Floater	3/4/4/6	50	0	10	12
Chryssalid	8/8/8/8	Melee	10	120	20	Hardened, Deals +2 damage on classic/impossible, Immune to stunning
Zombie	10/10/10/10	Melee	0	120	8	Deals +2/+4 damage on classic/impossible, Immune to stunning
Muton	8/8/10/10	70	10	10	12
Drone	3/3/5/7	60	10	Mechanized	12	Requires 'Drone Hack' arc thrower upgrade to 'hack'
Cyberdisk	16/16/20/20	70	10	Mechanized	18	Hardened while in closed form, Immune to stunning
Sectoid Commander	10/10/14/14	85	20	90	12	+25/35 will on classic/impossible
Muton Berserker	20/20/20/25	Melee	20	80	17	Hardened, Deals +1/3 damage on classic/impossible
Sectopod	30/30/30/30	80	30	Mechanized	12	Hardened, Immune to stunning
Heavy Floater	12/12/14/16	70	10	25	12
Muton Elite	14/14/14/18	80	20	20	12	+10 defense on classic/impossible, +10 additional aim on impossible
Ethereal	20/20/20/25	N/A	40	120	12	Hardened, +25/35 will on classic/impossible
Uber Ethereal	?/?/25/?	N/A	40	?	?	Hardened

Melee attacks always hit. Hardened reduces critical chance by 60% against the target. On Classic and Impossible, Aliens receive +10 aim and +10 crit chance bonus.

Alien Life Forms (EU2012)

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