Fading Suns and Flaming Heresies
This is the dark side of psionics. Urge has one main path, that of the Doppelganger, or Dark Twin. There are other paths, but they are rare and conditional.
Whenever a psychic fumbles a psychic power roll (or a Psi roll), her Urge stirs. For the next span, this dark twin may (or may not, gamemaster’s discretion) actively take part in the psychic’s life, in a manner dependent on the Urge level (see the list below). It may act in any way its level allows (using all lower level powers also) as many times as it wishes throughout the span.
We say “it” here, but it is really the character herself who is acting. Once she gains one or more Urge levels, the stress of her psychic powers has split her ego into two distinct personalities, one of which remains hidden in her unconscious. But with more power (Urge levels), this dark personality begins to grow, eventually becoming an entirely separate person.