From the Ashk Setting Guide

Istar was one of the three principle rulers of the primordial age, along with Bahamut King of Dragons and Tiamat the Dragon Queen.

Istar represented neutrality and watched the universe unfold.

Towards the end of the primordial age, Tiamat slew Istar in a battle which left the material plane scarred. Istar’s essence was condensed into one of his diamond eyes, which legends say can grant wishes.