Sly Marbo

He may very well be Primarch Corax disguised as a Catachan Guardsman. Reportedly, he can turn into a shadow-monster or a flock of ravens whose beaks can shred ceramite like toilet paper if he heads to the Eye of Terror. Heck, he even beat the crap out of Daemon-Lorgar once and probably kept the Chaos Legions from taking full advantage of the War of the Beast and the Age of Apostasy by carrying out assassinating key chaos lords and sabotaging hell forges!