Alien Overview (Apocalypse)
This section gives a brief description of the various aliens in X-COM Apocalypse and the threats posed by each. Enemies are sorted by their general order of appearance.
One of the most memorable and iconic of Apocalypse's many aliens, the Brainsucker is also likely the first you'll encounter. Reminiscent of the facehugger from Alien, these annoying, fast little critters will jump onto the heads of your Agents and "brainsuck" them, dying in the process. A brainsucked Agent is under permanent alien control; they must be subdued to complete the mission, and cannot be recovered. Brainsuckers will not target Androids or the defense guns of Security Stations.
The low health of Brainsuckers makes them only a minor threat in real-time mode, as your soldiers will probably be able to shoot them before they can complete their attack. However, in turn-based mode, only reaction fire can save Agents from charging Brainsuckers and they are as such a serious menace.
Note that there are two ways for Brainsuckers to appear; some are simply spawned at mission start like any other alien, but Brainsuckers can also be brought into battle by other aliens as Brainsucker Pods, which are fired at your troops with a Brainsucker Launcher.
The blue, bipedal Anthropod is the mainstay of the alien army, and functions as the alien equivalent to X-Com's soldiers. Anthropods have mediocre health and speed, and no innate attacks, but make up for it with the ability to use weapons and equipment. At the beginning of the game, Anthropods will only be equipped with Brainsucker Launchers and cannot directly damage your troops, but as the war progresses and the aliens begin to take X-Com more seriously they will be equipped with disruptor weapons, grenades and eventually missile launchers, while gaining access to shields and cloaking devices. As such, the threat posed by Anthropods is entirely dependent on their equipment loadout, and steadily grows throughout the game.
Unlike most aliens, Anthropods are quite vulnerable to psionic attacks with a Mind Bender.
Also referred to simply as "Alien Egg", these pink blobs are the first stage of the alien life-cycle. They cannot move and simply spit acid at X-Com Agents that enter the same room. Their spit attack is not particularly powerful and their health not particularly high, making them largely harmless; moreover, simply leaving Multiworm Eggs alone will result in their automatic live capture once all mobile aliens are subdued.
Another early stage of the alien life-cycle, Multiworms are large, green blobs of protoplasm that slowly amble about while spitting acid at your Agents. Their acid attacks are somewhat dangerous in the early game due to their decent accuracy and high rate of fire, but the Multiworms' biggest strengths are their high health (more than three times that of any other alien until the Megaspawn) and their death throes; when a Multiworm is killed, a swarm of four Hyperworms emerges from its corpse.
It is generally recommended that X-Com agents stun Multiworms rather than killing them outright; stunned Multiworms do not release Hyperworms, and at least one live Multiworm is required to access Toxin B. Note that a Multiworm must be brought to approximately 50% health before a Stun Grapple can subdue it.
Hyperworms are tiny, fast, weak aliens that slither up to your soldiers and whittle down their health with a constant stream of bite attacks. Their low health and lack of ranged attack makes them easy pickings at range, but if your soldiers are near a Multiworm when it bursts open it can be difficult to kill all four Hyperworms before they inflict serious damage. Fire and explosives are particularly effective against Hyperworms, due to both a damage-type vulnerability and these weapons' ability to hit all Hyperworms at once.
It is very rare to encounter a Hyperworm other than via killing a Multiworm, as they're not included in the standard alien rosters for UFO crews or infiltration teams; however, an alien force that's been hiding in a building for a while may include some.
Note that it is not necessary to actually capture a Hyperworm; while a live Hyperworm is an important research topic, four live Hyperworms are automatically obtained upon researching a live Multiworm.
Spitters are pink, headless bipeds that spit acid out of their open throats. Unlike Multiworms, Spitters are not particularly durable nor their attacks particularly fast or accurate, and as such they are simply fodder for an even moderately-equipped Agent.
Poppers are very fast, blue-green creatures that will run up to the nearest Agent and then self-destruct in a powerful explosion. This explosion can kill a full-health rookie through Megapol Armor, and as such Poppers are considered one of the most dangerous aliens in the game (particularly in turn-based mode, as you cannot simply focus your entire squad on the Popper as you can in real-time). Poppers will also detonate if killed by bullets, explosives, or bleeding out; this can be a bad thing if the Popper is almost on top of your soldiers, or a good thing if it's among other aliens. Another hazard from Poppers is that they leave a trail of smoke behind them as they run; this smoke is thick enough to provide concealment to whatever's following the Popper (including, potentially, other Poppers).
An intermediate stage of the alien life-cycle, between Hyperworms and mature aliens, the Chrysalis cannot move or attack. As such, these pose no threat whatsoever. Like the Multiworm Egg, they are recovered alive automatically if simply left alone.
These yellow, spindly aliens are basically alien special forces, and will be seen in increasing numbers as UFOs become more advanced. Like Anthropods, they use equipment, but they are much faster, have significantly-better accuracy, and can fly. Skeletoids with Personal Cloaking Fields are particularly deadly, as their mobility makes them hard to find. They share Anthropods' weakness to psionics to some extent, though not fully.
The Micronoid Aggregate is a puddle of green slime made up of billions of psionic micro-organisms. Due to their anatomy - or lack thereof - they have a very low profile and are resistant to many forms of weaponry. They also have no means of physical attack; their only threat is their psionic abilities. As such, the threat posed by Micronoids is entirely dependent on what X-Com brings to fight them; a squad armed with Toxiguns has very little to fear, as does a squad of Androids or trained Hybrids, while a human squad that's heavily-armed but poorly-armoured will have some problems.
Micronoid Aggregates are only found on board the Alien Bomber and in the Sleeping Chamber. Note that Stun Grenades have no effect on Micronoids, so Stun Grapples are the preferred method of taking one alive (psi-stun can also work, but requires a highly-trained Hybrid thanks to Micronoids' high psi-defence).
This behemoth of a creature is functionally an alien tank. Megaspawn are heavily armoured and have two inbuilt weapons: a high powered disruptor beam and a missile launcher with infinite ammunition. Due to the homing and area-effect nature of their missiles, Megaspawn are exceptionally dangerous enemies until X-COM Disruptor Armor is deployed late in the war. Fortunately, these will only be encountered when assaulting the last few UFO types or later alien buildings.
One quirk of Megaspawn behaviour is that they won't use their missiles at point-blank range (presumably to avoid killing themselves with splash damage). As such, an agent right next to a Megaspawn is often paradoxically the safest.
Another very large alien, the Psimorph is essentially a stronger, flying version of the Micronoid Aggregate. They have no exceptional resistance to weaponry, but don't need it as their health is the highest of any generic alien. They are also somewhat better at making psionic attacks. The same tactical advice applies as for Micronoids: deny them an opening by avoiding giving dangerous weapons to human Agents.
Psimorphs will only appear on the Alien Mothership and in certain alien buildings.
A much larger version of the Megaspawn, the Overspawn is not fought in the Battlescape but rather in the main Cityscape when it is dropped into Mega-Primus. It has no ranged attacks, and is thus a sitting duck for aircraft, but will rapidly tear down any buildings it encounters. This damages X-Com's score, but the extreme negative PR suffered by the aliens for such wanton destruction makes X-Com rather popular in the aftermath.
Obviously, an Overspawn near one of X-Com's bases warrants an immediate and forceful response, but otherwise the various corporation fleets that launch when it attacks their buildings are quite capable of fending it off.
The Overspawn cannot be captured alive.
The Queenspawn (there's only one) is the source of Multiworm Eggs and, therefore, all non-Micronoid aliens. She cannot move, but has a fairly-powerful acid spit attack and 600 health, as well as many, many guards.
Capturing or killing the Queenspawn is the objective of the Spawning Chamber mission. As there is only one Queenspawn, capturing her alive is generally preferred (and will unlock Anti-Alien Gas weaponry).
Obviously, these are not aliens, but you may end up fighting them anyway (specifically, if you investigate a building owned by a hostile organisation, if you raid a building, or if an organisation raids an X-Com base). As with Anthropods and Skeletoids, these are tool-users (obviously) and their threat level depends entirely on their gear - most human guns are fairly ineffective against Megapol Armor, but humans with Marsec Heavy Launchers or Devastator Cannons are a very different kettle of fish (note that human enemies are more accurate than Anthropods or Skeletoids). In the final stretch, they'll even acquire Personal Disruptor Shields and Personal Cloaking Fields, which given their resistance to the Toxigun makes them among the most resilient of opponents (they also tend to be found in huge numbers). On the plus side, they cannot fly and will never get access to the horrifying Entropy Launcher.