A hideous, plated skull with gaping beak.

Although they aren't anything special in terms of defense, a knollworm skull is an excellent find. Characters with mental mutations will likely want the +1 ego boost to improve the effectiveness of their abilities, but all characters can benefit from ego's impact on trading - simply equip the skull before speaking to a merchant, and all your trades will be made at a discount.

Knollworm Skulls can be obtained from the corpses of Knollworms using the Butchery skill; they're also commonly found elsewhere, especially worn by enemy espers.