As seen around 3E 427
The Ring of Phynaster was made by the Hero-god of the Altmer, Phynaster. With the Ring and a new walking technique he discovered, Phynaster was able to live for hundreds of years. The Ring boosts the wearers resistance to poison, shock damage, and magicka. The Ring was also cursed by Phynaster so that it eventually disappears from its holder's possession.
In 3E 427 the Nerevarine found the Ring in Senim Ancestral Tomb on the corpse of Pop Je.