The Thonos was a demon, summoned to Sanctuary to do the bidding of the Triune cult and it's master, the Primus. The creature is mentioned in the second novel of Richard A. Knaak's Sin War trilogy, Scales of the Serpent.
The Thonos is described as being an oval-shaped demon the size of nearly twelve mortal men, hiding under the ground most of the time. Hundreds of tentacles of various sizes and lengths sprout from the oval mass. Any area on the Thonos' body that was not covered by tentacles was covered by eyes the size of a human's head, but otherwise very similar to the eyes of a human. The mouth was situated underneath the creature: it was a beak-like projection with rows of sharp teeth and a thick tongue.
During the final battle between the Edyrem and the Triune army in the jungles near Kehjan, the Thonos emerges once more. Uldyssian faces the demon and is nearly torn asunder when Achilios appears. Blessed with holy power, the archer makes quick work of the vile creature.
Though the Thonos was described as a subterranean creature, in its final moments the demon triesd to flee over land. Blinded and broken the Thonos eventually collapsed on top of the Triune army, crushing both Peace Warders and morlu.