Outsiders are a type of alien construct that appear to be humanoid yet contain no organic matter; Dr. Vahlen describes the first one discovered as a "being of almost pure energy." Outsiders materialize out of glowing orange crystals and are only encountered inside crashed or landed alien UFOs. They are typically located in the core or the command room of the UFO and only appear when XCOM soldiers get close enough to spot their crystals. Originally believed to be dedicated guardians for the alien vessels, later research reveals that these Outsiders may actually be some kind of mobile long-range interstellar communication system.
A slain Outsider will disintegrate completely, leaving no corpse; on the other hand, stunning it with an Arc Thrower will leave behind its crystal core.
The crystal will only materialize when an XCOM soldier gets in visual range. Use this to your advantage by neutralizing all the other aliens first.
Be careful when approaching Outsiders with unarmored troops. Their high base accuracy combined with the additional boost of the Light Plasma Rifle means Outsiders can, given the opportunity, devastate a low-tech team.
A possible bug may occur where an Outsider is captured, but the research option will not become available. This may be caused when you capture the Outsider as your first "live" alien. Interrogating any live alien will allow you to research the shard.
Although possible, it is not necessary for research purposes to capture more than one Outsider, as the Outsider Shard cannot be sold on the Gray Market. However, stunning multiple Outsiders will allow the intact recovery of their Light Plasma Rifles which can be used once the associated research is complete. With proper planning, Outsiders can be relatively easy targets to capture given their predictable spawn points and low health. This particular tactic may be employed to great effect to obtain plasma weaponry cheaply and safely, especially when the "Marathon" Second Wave option is enabled, due to the increased amount of encounters with this type of enemy.
The We Have Ways continent bonus does not apply to the Outsider Shard as the shard is treated as an item instead of an alien captive which in turn means the Alien Containment facility isn't required to store it.
Once you have raided the Alien Base, Outsiders will disappear from the game. Sectoid Commanders (and later lone Ethereals) will take their place.
Outsiders are the only aliens not mentioned or spawned by the Uber Ethereal during the Temple Ship Assault – a curious omission as far as in-universe storyline is concerned – as other robotic/mechanical units such as the Cyberdisc, Drone and Sectopod are represented. A plausible explanation is that the beings known as "Outsiders" were, in fact, merely projections of the Sectoid Commander who, hiding in the Alien Base, used Outsider Shards together with the Hyperwave Relay to safely coordinate operations by proxy through its Psi powers, without actually endangering itself. Another possible explaination is that they were some form of AI connected to the UFO that filled a command role until other Sectoid Commanders were deployed, since the possiblity of them being a long range communication system would allow them to relay information back to their commander and recieve new orders directly. A third possibility is that they are avatars for the Ethereals.
It is not known how exactly did the Light Plasma Rifles used by the Outsiders materialize when the Outsiders appear.