XCOM-EU Mission - Alien Base Assault.jpg

The Alien Base Assault is an essential mission in XCOM: Enemy Unknown. This mission carries a randomized Operation designation.


This mission becomes available after crafting the Skeleton Key from the Outsider Shard. It can then be initiated at any time from Mission Control.

Thanks to Dr. Vahlen's research, and some clever developments down in Engineering, this "Skeleton Key" should allow you to breach the alien compound. We've only got one chance to make this work though, so let's make sure we do it right.

Central Officer Bradford, XCOM: Enemy Unknown

Mission Brief[]

We have gained access to the Alien Base by means of the Skeleton Key. Use extreme caution; analysis indicates the presence of a large combined enemy contingent. This mission cannot be aborted.

– Mission Brief, XCOM: Enemy Unknown

Mission Objectives[]

▪  Penetrate the base's defenses
▪  Neutralize all invader forces
▪  Identify and capture the base commander, if possible
– Mission Objectives, XCOM: Enemy Unknown

Mission Site[]

L-shaped base layout

Upon launching the mission, the Alien Base appears to be randomly selected between two layouts. They can easily be distinguished by the loading graphic, as one features an L-shape (see photo, right) while the other does not (see photo, above). Both layouts consist mostly of pathways with guardrails that serve as cover. The base features a multi-elevation environment, which can benefit the player if soldiers are positioned at higher vantage than the enemy.

Multiple places in the base have vast spaces with full cover available, making it easier to perform frontal assaults with close-combat units, such as shotgun-equipped Assault Class soldiers.

Enemy Types[]

Depending on the stage of the alien invasion before initiating the mission, it is possible for the Alien Base to feature every enemy type, with the exclusion of Ethereals and Outsiders.

22 enemies total on Easy, Normal and Classic difficulty, and 28 on Impossible difficulty.


Soldiers infiltrate the Alien Base

The Alien Base has a limited number of enemy troops which are scattered throughout. Some of them are mobile and actively roam the map. In most areas, it's preferable to advance carefully, keeping to cover and using Overwatch. Enemy aliens will most likely be hiding behind doors that obscure vision, so take care to line up your squad in preparation for a door breach. The multi-level environment offers opportunities to take a high vantage point, gaining an advantage in Aim and Defense over the enemy troops. On the other hand, the numerous bulkheads and multi-level gangways also obscure vision making it possible for to run facefirst into alien patrols.

The mission culminates in an encounter with a single Sectoid Commander, which, regardless of the stage of alien invasion, is the first encounter with this enemy type.

Capturing Sectoid Commander[]

It is imperative to capture the Sectoid Commander alive for interrogation in order to obtain the Psionics research credit. The Sectoid Commander is isolated when encountered during this mission, simplifying the task of capturing it, whereas in future missions they are rarely encountered alone.

In addition, the research credit applies to a project involving the Hyperwave Beacon found at the base, which becomes available immediately after the assault. Soldiers equipped with Flush, Disabling Shot, Rifle Suppression, Shredder Rocket, Flashbangs, Ghost Grenades, Secondary Heart, Mimetic Skin and Neural Damping helps avoid the Sectoid Commander's Mind Control ability and capturing it significantly less difficult than it needs to.

Mission Debriefing[]


XCOM was unable to eradicate the alien base in <Continent Name>!
– Council Transmission, XCOM: Enemy Unknown


XCOM successfully infiltrated and eliminated the alien base in <Continent Name>!
– Council Transmission, XCOM: Enemy Unknown


This mission serves to advance the main plotline, and reduces panic by two points worldwide.

In addition, it provides a substantial reward in the form of loot. Aside from alien corpses, Weapon Fragments, and any intact weapons from enemies captured alive, the base is looted for artifacts, some of which cannot be found elsewhere:

Quantities may vary. Most artifacts can be damaged by gunfire or explosions, serving no purpose other than to be sold at the Gray Market at a discount. The one piece of alien technology guaranteed as a reward is the Hyperwave Beacon, as it cannot be damaged.

XCOM: Enemy Within[]

  • Worldwide panic is only reduced by one point on Classic and Impossible difficulties.
  • The aliens will retaliate by assaulting XCOM headquarters 15-20 days after this mission.
  • Approximately 50 units of Meld are automatically recovered following the mission, however there are no canisters to be found.


  • It may be advisable to unlock this mission and build the key as soon as possible. When faced with a difficult situation in regard to panic levels, complete the mission for the worldwide panic reduction.
  • Despite Bradford's statement that "we've only got one chance to make it work," the mission can be attempted again following a failure if the entire squad is killed in action.
  • The topography map in the mission briefing appears to show the alien base to be located somewhere in Bactria, though on the actual hologlobe the flight location will be semi-randomized, including Colombia and Siberia.