Zachariah attacks the Saint Petersberg Palace, kills the Empress and Castiel's father, and as many other angels as possible.
Dean Winchester helps Castiel and Gabriel to escape.
While chasing Castiel and Gabriel, Zachariah falls through the ice they're running accross and dies.
Azazel assumes power over the Zacharists
Castiel loses Gabriel, recieves head trauma, and loses memory. Becomes homeless.