One in a Trillion
Adeptus Templar Psykologis
Getting into the head of your enemy, is to make an enemy of yourself. The Temple Psykologis are said, by those aware of their presence, to be limited to 13 missions before they are considered to have been in the minds of too many dangerous foes, and are subsequently required to present themselves for execution upon completion of that last mission.
Only one, in their entire history, has not done this.