Yig, the Father-Rat of Serpents, wasn't initially insane. Quite the opposite, Yig was a very mundane Skaven. But his association with Bokrug got him thrown into Arkham Sanitarium. His time spent in that lock up lead him to believe he had given rise to all snakes and snake-like people. He collected a small following of Lizardmen, whom he used to help his 'fellow gods' break out of Arkham Sanitarium. He now uses Blood Bowl as a way to unify the lizardmen and snake people under his cult.