As far as I remember, black mages of Amdapor summoned Diabolos and the white mages of Amdapor, seeing that disaster, put the voidsent to sleep and since the city was in ruin they might have allied with the scholars of Nym to defeat the black mages that they thought to have come too far. Maybe the MNK quest giver have some info on this, I never heard of the alliance between the white mages and the scholars before too...