Eons ago, a group of aboleth left the Material Plane to wander through distant planes—seeing them through magical scrying was not enough, so these aboleth used astral magic and bodily travel to see far beyond normal realms.
A Forgotten Tribe. As ages passed, memories of those who departed slowly faded from the minds of those aboleth who remained behind. Those few aboleth who did remember that long ago some of their kin had gone plane-wandering assumed that the wanderers must have died in distant hells or paradises.
Changed by Planar Wandering. The plane-wanderers hadn’t died. Instead, their eons-long exposure to alien realms and to the space between changed them, restructuring their life force and making them into something even more nightmarish—but better able to withstand both strange hells and golden realms of eldritch delight.
Servants of the Void. They returned even more corrupt and powerful than they had left, and these wandering nihileths returned to the mortal world intent on spreading the influence of the Void and the utter evil they found in the vast darkness between worlds.