Legend has it that he was created by the destruction of the twelve Lords of Decay that sat upon the Council of Thirteen. Much to the annoyance of the Great Horned Rat they pledged to work together on a certain scheme and some quietly squeak that it was against their patron deity. [2a]
For long millennia did their god cast them into a hideous shadow oubliette and tormented them. Eventually he released them but now combined into a single form - a truly mighty Verminlord that not only possessed the knowledge of each clan but also was loyal to its god. [2a]
His knowledge includes the Arcane, Fleshcrafting, Plague-brewing, Shadow-slinking, Warrior skills and Warp-tech. He can also can cast The Dreaded Thirteenth Spell that mutates his enemies into Skaven and wields a Doom Glaive and a Plaguereaper into battle.