Missing Information (Issue: Location picture(s))
This article requires more information about the subject. You can help Legacy of the Dragonborn by expanding it.
Dragonsreach Dungeon, in a an iron pot in the tunnel directly below one of the cells. The tunnel is accessed through a loose grate with a dead body lying on top of it. The loose grate is in the cell you end up in when you are taken to jail.
There's a note in the pot with locations of some other rings - Warlock, Vampiric, Mentor, Phynaster.