A large cube shaped section of skin and conjunctive tissue. There's a maggot inside it but it's not known if that's a normal part of it or the ailment. The development of maggots under the skin is called myiasis, hence the name Myiasis Cube. The cube's only way of communicating is by jiggling intensely.
In Awful Hospital as a patient.
The Myiasis Cube is based on common illustrations of cross-sections of skin found in medical textbooks and journals.