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