"The hallucinations are getting worse. I saw a giant made of water today. This apparition coughed up a seething mass of crustaceans before it disappeared beneath the waves. I watched these things dismember and consume everything they encountered. I have named them amputators, even though I am not sure they are real."— The Crazed Man(src)
As with other small foes, Amputators usually appear in large numbers, and sometimes accompany Flesh Harvesters. They swarm the player, attacking in melee, but otherwise have no special abilities.
Normally, they are only found in Tidal Cave, but can be summoned to other areas of Greenhollow (anywhere near the open water, most notably the bridge that divides the island). A huge creature conjured of pure water (most likely by Abyssals) appears and, after a brief delay, spits out a dozen or two of Amputators.
The Helm of Cranial Crustacean is fashioned in form of a small Amputator.