Old gods don't have avatars. Infact, the old gods ARE avatar's/creations of the Void Lords....
Also, The cataclysm did NOT start cause of N'zoth, or deathwing. It started with the old gods dying in general. The Old god's deaths disbalanced the elements, created a nightmare on azeroth, and shook up both N'zoths bindings, as well as deathwings...