The Beast

The creature known simply as "the Beast" was recovered on a nameless world on the fringes of the Adrantis nebula. It hid there, posing as a simple refugee, together with thousands more that had fled the destruction of Calixis. Barotta and his men captured it, but soon found out there was nothing simple about it. Rather, it was perhaps the most fearsome assassin ever created.

The Beast seems to have once been a human, and even claimed to remember fragments of a life before. According to the Beast, he was born on Scintilla, where he lived a life of crime. He was taken by the Abrites and sentenced to a life of toil. Instead, he ended up in the clutches of the Holy Orders of the God-Emperor's Inquisition. Using arcane techno-magic, Xenos transfiguration engines, and the darkest warp-craft, he was remade into a deadly assassin.

How much of this is true? No one can know for sure. His memories were fragmented, confused, contradictory. Some of them were undoubtedly implanted, designed to either control the Beast or enable him to operate undercover. It could be that he was no memory of his original life, if ever he had any, but only bits and pieces of the lives he has assumed.

The Beast is a shape-changer, able to take any humanoid form, including that or a Space Marine and an Ork. He can summon fell warp energies to tear apart starship bulkheads and armored Astartes with equal ease. He disappear into shadows and walk through walls. It is incredibly hard to kill, and quickly regenerates even mortal injuries, but can be incapacitated and immobilized by extreme use of force. 

The Beast also has a "rest" state, where he is a docile, featureless, and vaguely human creature with no face or gender. When in the rest state, he can be given orders to kill. The creature will then use its considerable skills and powers to find the target and eliminate it. Then it will return to its master and resume the rest state.

The Beast is currently on Vaxanide. After assuming the form of an Ork, it killed the Ork Warlord assaulting Vaxanhive, but didn't return and revert to the rest state. Instead, it became the next Ork Warlord, and the Ork horde readily flocked to its banner. It was last seen armed with an Axe of Khorne, in addition to its usual warp-klaw.