Overviews of Aliens
Any budding X-Com commander should be aware of the basic details of the threat facing the Earth, to this end an Overview of Aliens can act as a first step in preparing the novice for their first clashes on the battlefield. Once the commander is conversant with this information further details can be found by looking at aliens' specific pages or Alien Stats.
Low Threat: Sectoid
The Sectoids are usually the first alien you'll see. They resemble the "Greys" spoken of by New Age types and the X-Files television show: short humanoids with pale skin, elfin features and large, almond-shaped eyes. They are the only weapon carrying life-form that chooses to go naked into battle. This possibly psychological tactic does not appear effective, since it neither promotes a berserker style morale on their own part, nor do the sectoid size dangly bits appear to intimidate X-COM soldiers.
Sectoids are physically weak in combat. They have average mobility, no armor worth mentioning, and will fall easily to any weapon. Without their psi equipped Leaders and Commanders, they can be overcome with minimal risk. Be aware that Sectoids are on average more accurate shooters than floaters, and their ability to react is second to the Ethereals.
However, on any mission that doesn't involve a small/very small-sized UFO (Small Scout, Medium Scout, or Large Scout), the Sectoids will have a Sectoid Leader in their ranks. This Leader is capable of Psionic attacks that can panic your troops or even bring them under alien control. In the early game when Sectoid sightings are most common, this can be devastating because X-COM has not yet researched psionics, and so is wide open to this type of warfare. The advantage for X-Com is that there will be a limited number of Leaders or Commanders in combat, and capture of either will allow research into the field of psionics.
On any Mission involving terror units, the Sectoids will be accompanied by the formidable Cyberdisc, which adds another degree of complexity and difficulty to the situation.
Low Threat: Floater
The Floaters are the other variety of aliens you'll often see early in the game, sometimes before Sectoids. They are purple humanoids clad in capes, floating perpetually above the ground.
Floaters are slightly tougher than the Sectoid, with slightly enhanced armour and health. Most of the floater ranks aren't as accurate or as reactive as Sectoids, with the exception of their Leader and Commander classes. Floaters ranked at Soldier, Navigator or Medic are able to remember enemy locations one turn longer than Sectoids of similar ranks.
Floaters, as per their name, are capable of floating in mid-air. They use this ability on some occasions and you will often find them hovering on rooftops and other elevated positions. They tend be more prone to staying in upper levels of some buildings. They will rarely make good use of cover while floating above ground level.
Floaters have no psionic capability, and their terror unit, the Reaper, is weak. Floaters arguably make for the easiest large UFO missions, terror missions and alien base attacks due to these weaknesses.
See also: Floater | Low Threat: Reaper - Terror Unit
Low Threat: Reaper - Terror Unit
The Reaper is the dedicated Terror Unit of the Floater, and will only be seen accompanying them. It is a large, furry mammalian biped of size comparable to an X-COM HWP. It is armed only with its bite, which obviously can be used only when the alien is directly adjacent to its foe. It is arguably the least threatening alien in the game as long as precautions are taken and they are not allowed to get close to you.
Since they are large, reapers are easy to spot, make big targets and will not be able to follow you through small gaps. Their large size allow area-effect weapons to deal more damage then usual.
Reapers have high Health, but are only moderately armoured and have no resistances. They are vulnerable to fire, and as a large unit with little Under Armour and no resistance High Explosive munitions are also extremely effective.
Low Threat: Silacoid - Terror Unit
The Silacoid is one of the two varieties of terror units deployed with Mutons, and will only be seen accompanying them. It resembles a large pink rock that slowly rumbles along the ground and leaves a trail of burnt terrain in its path. Their only combat ability is to make a melee attack, presumably involving the burning heat of its body.
Silacoids have lacklustre mobility, and their burn trails make them easy to track. They are well protected and can absorb a moderate amount of fire. They have the smallest profile of all the aliens, making them harder to hit.
Considering that they are usually only encountered in the mid- to late-game, the sight of this underwhelming combat unit comes as a pleasant surprise to Commanders accustomed to more fearsome prey. They are less of a threat than Reapers and can be easily out-walked.
See also: Silacoid | High Threat: Muton
Medium Threat: Celatid - Terror Unit
The Celatid is one of the two terror unit types deployed with Mutons, and will only be seen accompanying them. It resembles a large, pink kidney bean that hovers along slowly at ground level. It is better protected than some aliens and has good mobility. It has terrible reaction ability, but makes up for it with high firing accuracy.
It attacks by spitting venom at its target. This venom is extremely dangerous to unarmored units. It can only project its venom over a short range (roughly 7 squares), but is able to fire it in an arc. This lets it shoot over low fences.
The Celatid is at home in close quarters, particularly indoors where it can pop out from around corners, and its small size can make it hard to hit. However, with proper care taken, it is only a moderate threat on the battlefield.
See also: Celatid | High Threat: Muton
Medium Threat: Cyberdisc - Terror Unit
The Cyberdisc is the dedicated terror unit of the Sectoid, and will only be seen accompanying them. It is a mechanical floating disc, comparable in size of a HWP. It has moderate mobility, and can float at any level above the ground. It attacks with a devastating plasma shot, capable of firing up to three snap shots in a round with good accuracy and reactions superior to X-Com HWPs.
It is resistant to armor-piercing weapons, and holds up well against psionic manipulation. Its slim profile can make it a difficult target for an object of its size. Worse, when it is destroyed, it usually explodes with a ferocity comparable to Alien Grenade or worse. New commanders often have difficulty destroying it with Terran weapons; it is advised to direct as much firepower as is available against one. Advanced weapons are more suitable for destroying it.
Commanders should remember that any mission involving Cyberdiscs will also involve Sectoids with psionic capability. However, not all missions with psionic Sectoids will involve Cyberdiscs.
Medium Threat: Snakeman
Snakemen can make an occasional appearance early on, but won't show up in force until three months into the game. As their name suggests, they are reptilian, resembling snakes with arms, slithering along on their lower bodies. They have noticeably better armor than floaters or Sectoids, and are capable of absorbing more damage than either. Their firing accuracy is second to the Ethereals, making them dangerous marksmen.
Their key flaw is their low mobility. Despite a terrain move advantage they don't have a lot of TUs and have less stamina than most aliens. Snakeman also have terrible reactions, making them easy to ambush.
While seemingly a weak enemy on their own, large Snakeman missions involving Terror Units can be truly fearsome with the inclusion of their devastating Chryssalid support units.
See Also Snakeman | High Threat: Chryssalid - Terror Unit
High Threat: Chryssalid - Terror Unit
The Chryssalid is the dedicated Terror Unit of the Snakeman. One of the most feared enemies of X-COM, these creatures have shiny black exoskeletons, an insect-like look, with a grinning skull. They resemble the art of H.R. Giger. They have incredible mobility, lightning-fast reflexes, and can absorb large amounts of fire. These abilities allow them to slug it out long enough to deliver their chief threat: their infectious bite, which can penetrate even the thickest armor and can destroy HWPs. Humanoid targets are turned into zombies.
The ability of a single specimen to turn an entire troop of X-COM operatives into a new population of Chryssalids is the stuff of a Commander's nightmares. Killing Chryssalids should be your top priority whenever they are present. Concentrate fire upon them. Don't be afraid to sacrifice any civilians or X-COM troops nearby-- if the Chryssalid doesn't die, they'll probably be infected anyway. Ensure that all Chryssalids that drop are dead, not merely unconscious. Listen for the lack of a death-cry - do not hesitate in blowing up its body or reviving the Chryssalid with a Medi-Kit to shoot it again.
Any X-COM trooper or civilian bitten by a Chryssalid will turn into a zombie -- a drooling humanoid that walks around at a slow pace and tries to launch powerful melee attacks against your troops. The creature itself is easily killed. Don't let them stand next to you for long. By themselves, they are quite harmless unless they are allowed to get too close. They can be easily outpaced as they are not able to move very far.
Unless zombies are burned to death with an incendiary attack, the zombie's real threat is that a Chryssalid will hatch out of its corpse when is it killed. The fresh new Chryssalid can turn more people into zombies. Yes, this means that a single Chryssalid can turn every human on the map into a Chryssalid.
High Threat: Ethereal
Ethereals are the ruling caste of the Alien hordes. They will not be seen until June, but when they do, expect them to be armed with the most powerful weapons in the alien arsenal. Their bodies are thin and emaciated, with pale, pinkish skin. Stretched Sectoids. In combat they wear billowing orange robes which make their narrow frames look more imposing, and it provides them with most defence of the main races. This makes them comparable with Mutons despite having less health.
They can endure large amounts of damage and have very good combat skills. They also have good mobility and are able to fly by walking through the air. They will do this with little hesitation. Every Ethereal has psionic abilities; expect psionic warfare to commence almost immediately in any combat against them. Always engage Ethereals with the greatest caution, and all knowledge of psionic combat you have at your disposal. Capture of an ethereal of any rank will allow research into the field of psionics.
As if their combat skills, high mobility, resistance to fire and psionic ability were not enough, they also have one of the strongest terror units in the game-- the Sectopod.
High Threat: Muton
Mutons are the shock troops of the Alien invasion, possibly the beefiest combatants you'll go up against. They will start to appear from April. Expect them to be packing the most powerful weapons in the alien arsenal. They have purple skin, but everything but their faces are covered with green skin-tight armor.
They are lightly armoured but have lots of health to absorbing heavy fire. They are especially resistant to armor-piercing rounds. They have respectable reactions and mobility, and average accuracy. Unlike other aliens, Mutons have a limited set of ranks, Soldier, Navigator and Engineer. Mutons of all ranks have high aggression.
They are not without their weaknesses, however. They are known for a short attention span, and will forget the presence of enemies beyond their sight more quickly than other aliens would. They are vulnerable to psionic attacks, and have no psionic talents of their own.
They are the only species in the Alien invasion with two varieties of terror unit, the Celatid and the Silacoid. Neither of these creatures close to matching the Muton's combat prowess.
High Threat: Sectopod - Terror Unit
The Sectopod is the dedicated terror unit of the Ethereal, and will only be seen accompanying them. They are large, bipedal robots that are equal if not bigger than HWPs. They are the most heavily armored units of the Alien horde, resistant to almost all forms of damage including even alien weaponry, requiring heavy fire to destroy one. Its only known weakness is to laser weapons, and even against those it can sustain a good degree of damage.
They attack with a green-laser weapon that is capable of automatic fire. They are the only alien unit to use laser type weapons. They can fire with a high degree of accuracy and reactions far superior to HWPs. They also have good mobility.
Some commanders keep special weaponry on hand exclusively to deal with them, including the Tank/Laser Cannon and the Heavy Laser. Blaster Launchers are effective but direct hits are required and occasionally may require several hits. Stun Bombs are also quite effective against weakened or beginner difficulty Sectopods.