Komus is the enigmatic Tyrant Star that forms the centerpiece of much of the fluff of Dark Heresy's first edition, that takes the form of a black white dwarf. While nobody knows exactly what it might be (read: its fluff is deliberately vague so GMs can decide what it really is), insanity and destruction always follow its appearances. This destruction usually takes the form of volcanoes, groundquakes, mass mutations, and a fuckton of psykers being born. Visions speak of stars being snuffed out by it in the possible future. Oh, also, daemons are frightened by it. It's speculated to be as old as the universe itself. Basically 40K's take on the Schwarze Sonne/Black Sun occultism. As expected, it's cranked up to insane levels of power and nastiness.
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. We know that he was trapped in the Star, and he may or may not be directing it, thanks to the amount of signs and quotes associated with him that pop up in visions of the Star. It's equally likely that the mad trader has become the "face" of Komus. Whatever the case, all signs point to the Tyrant Star making things rough for the Calixis Sector in the future.