Deals irresistible mental damage
Impervious to physical damage
Disappears after 5 turns
Doppelgangers are hazy psionic apparitions that superficially resemble a cloaked copy of their target.
Doppelgangers are first encountered on Silent Isle, after being hit by a Strange Feeling a few times. They cannot be damaged by any conventional means, only by mental psionic damage. However, they disappear after 5 turns.
The player can also summon doppelgangers of human enemies with Bilocation psionic ability, which powerful mind controllers may learn from the Mysterious Pillars in Silent Isle (requires 7 will and Psi Empathy).
High level enemy thought controllers such as Lunatic Psychopaths can also summon doppelgangers of the player.
They are otherwise very similar to their Silent Isle counterparts, but their damage increases with the player's effective Thought Control skill.
Players working for Praetorian Security will encounter a unique Ancient Rathound with psionic abilities named Jookhela when sent to investigate the Sewers generator. Among other abilities, Jookhela has access to a unique version of Bilocation which allows it to create psionic duplicates of itself, instead of the player. These duplicates, besides looking like Ancient Rathound silhouettes rather than humanoid silhouettes, deal more damage and have more health than normal Doppelgangers.
Another type of Doppelganger is encountered much later in the game, within the Tithonus Laboratory. It is implied that these ones are actually the disembodied consciousnesses of several test subjects kept in suspended animation as part of a Biocorp experiment involving fragments of the Mysterious Pillars (Deep Caverns) found past the Labyrinth.
Unlike other Doppelgangers, these ones aimlessly wander the facility after the suspension cells are turned off, will attack the player if they are detected, and each has a name corresponding to their counterparts in the suspension cells. Although they can be "killed" the same way as ordinary Doppelgangers can, they will endlessly re-spawn until the cells holding their physical bodies are disabled outside of combat.
Doppelgangers have low health, but they resist almost all forms of damage, and can only be directly harmed with Neural Overload. They have True-Sight, thus cannot be affected by Neurovisual Disruption. They are immune to most status effects, but not to fear from Frighten, and can be incapacitated with Mental Breakdown.
Technically they can be intimidated with Yell, but since they don't rely on offensive skills this won't have any benefit. Enrage works on them, but it is similarly useless, since Doppelgangers don't change targets. While Bilocation can also be successfully used, Doppelgangers fully resist damage done by Doppelgangers.
Although Locus of Control gives immunity against Bilocation, it does not protect against the mental damage of Doppelgangers. Resolve reduces damage received from doppelgangers from version 126.96.36.199 onwards.
Even though the damage they deal targets the mind and so bypasses shields and armors, it is delivered through normal melee attacks and can be dodged.