Dagannoth are sea monsters. Many are found near areas inhabited by the Fremennik, who call them daggermouths and hunt them for their skin, which is used to make armour. There are many types of Dagannoth, the most powerful being the three Dagannoth Kings. Dagannoth are assigned by Chaeldar, Sumona, Duradel/Lapalok and Kuradal as Slayer tasks. The Balmung, released with the quest Blood Runs Deep, hits them very hard, similar to the effect of using Keris on Kalphites or Scabarites.
Little is known about the origins of the dagannoth species. Dagannoth are first documented in Third Age where they started attacking the Fremennik, causing great losses to them. The Fremennik people battle them to this day.