The Guise of the Dead is composed of the flesh of an undead creature (this can be any race but it must be humanoid). This creature is skinned and the skin must be treated with special potions and rituals to preserve it’s undead nature. It must also be routinely taken care of with a special herbal mix to retain it’s elasticity and moisture.
The first of these was created by a necromancer whose fixation with the undead lead him to create something so that he could walk amoung them, not as master but as equal, and really get to understand them. This same man wrote many books on the subject of undead “culture”.
Only those people with a special disposition (alignment) would ever even consider using such a grotesque item.
The wearer pulls the skin over their own. The Guise then molds itself to the wearers shape and seals itself closed. For all intents and purposes, the wearer will resemble the race of the creature their guise is made from, an undead member of that race.
Any undead creatures encountered will ignore the wearer, sensing them as “one of their own”. The effects of the Guise will wear off after a while, the skin will begin to unseal itself and they will eventually be exposed for what they are. The overall lifespan of the piece is a few months up to a year, depending on wear-and-tear and the dilligence of maintaince.