The Deepcoiler was a huge underwater serpent that lived in the Great Inland Lake. It was said to be more than three boat lengths in size. Shrews thought that the creature was only a legend, but found it to be a frightening reality when it revealed itself.
The beast attacked many times at random during the recovery of the Blackstone, and it caused the deaths of many creatures including Spriggat, Dethbrush, and a number of GUOSSSOM shrews. It also nearly killed Tubgutt.
The Deepcoiler was killed by Samkim after he thrust the Sword of Martin into the roof of its mouth. When the Deepcoiler tried to close its mouth, it pushed the sword up into its brain. Its body later washed up on shore, where it was found by Mara, who retrieved the sword. Mara later met up with Samkim and returned the weapon to him.
The Deepcoiler was later referred to as a "lake monster" by Ellayo Swifteye when she told Friar Butty an old story about Marlfox Island that she heard from her father.