Been a few years since I ran it, but here was my take.
The Ishru were embodyments of psychic energy, feeding and fueled by the emotions of sentient life. They were accidently created by an Althan scientist who was experimenting with psychic limits and capacities of the various slave races. Originally a single entity of psychic energy, the battle to contain/destro the creature fragmented it into numerous aspects, embodying the best and worse of emotions. When fragmented, they fled, their minds broken and primal, into the wilds of Kulthea..
Eons pass, and the origin of these elemental psychic beings is long forgotton. This leads to the story presented in the adventure. The lord of malice and what not were the forms the ishru took when free in the world, before being trapped by the kirzawhachamacallit. When free, they assumed forms fitting their focus, which I had as a family or racial line. I picked the Kuluku and Anzeti to create legends around regarding the mostly monstrous feats of the ishru when free.
Upon their release the Loremasters did not get an army like they imagined, but the Ishru merged with the races they use to haunt. The result was the emergence of psychic powers across Kulthea and a Hero that would stand against the Unlife and kill its Avatar...and unite the dragons...and ressurrect a dead god...but those are seperate adventures.