Komus
Komus is the enigmatic Tyrant Star that forms the centerpiece of much of the fluff of Dark Heresy. While nobody knows exactly what it is (read: it's deliberately vague so GMs can decide what it really is), insanity and destruction always follow its appearances. Oh, also, daemons are frightened by it.
More recently, it has been connected to the Haarlock dynasty of Rogue Traders, the last of which went insane after his wife's death and seeks to use the Tyrant Star's power for...something.