Dagannoth Rex is the Melee-based Dagannoth King found in the depths of the Waterbirth Island Dungeon, along with fellow Dagannoth Kings, Dagannoth Supreme and Dagannoth Prime. Rex is the only monster to drop the coveted Berserker and Warrior rings, which can be sold for hefty prices. He, along with the other Dagannoth Kings, can also drop the rare dragon axe.
If players venture too far out west to lure Dagannoth Rex, they risk attracting attention from both Dagannoth Prime and Dagannoth Supreme, namely Prime since he wanders closer to the east and north-east. This can be avoided by staying in the southeastern part of the arena, as Prime will rarely (if ever) go there, and attack Rex only when he's nearest to the east.