Well there is really no stories written on that yet...
Basically the great destruction occured after Micolon's (Negav's former king ) plot against nagas failed and backfired very badly ( as described in the novel "Crisis and scarecrow ). A dark time for Negav followed, during which different groups took power. The one who had the last word in the end was the Psi'ol magiocrats.
I know this is vague but I haven't worked very much on that yet ^^;