The ring is currently held by Vindamea Drethan in Ashirbadon, a cave on a small island east of the ruins of Bal Fell, just east of the city of Vivec at the bottom of the map. Indeed, just head directly east from the ruins and it is on the first island you reach.
The cave is small though filled with several tough creatures possibly including atronachs, dremora, and daedroth. Vindamea is again a high level mage wielding the Warlock's Ring and can be tough to take out. Don't forget to check for treasure once you've killed her.
If your disposition with Ajira is less than 70, she will not give you this quest. Instead she will tell you to come back when you are a Warlock even if you already are.