The Seers' ring is one of the Fremennik rings and is dropped by the Dagannoth Supreme in the Waterbirth Island Dungeon. The Fremennik rings are some of the very few rings in Arigon to give stat bonuses, and the Seers' ring is considered to be the best ring for mages to use.
It can be enchanted into a Seers ring (i) Seers' ring (i) by using a Ring of scrolls, obtainable from the Slayer Rewards