This is a morcid, also called a death anchor, it's a small wearable trinket made by someone to seal a pact with Venmortsen, god of death.

The quality and material doesn't actually matter, it is used up once the user dies.

The quality of the resurrection is directly linked to the importance of what was given up for a pact.

Something of low importance will res the user degraded power, sending their level back very far, as well as taking longer to res, meaning they rot more, possibly being a skeleton.
Weeks, months.

Something of greater importance results in a faster res, resulting in less notable undead features, and being set back less in power, a few days to immediate.

Memories, body parts, belongings, or if you're evil, other people can all be used in a pact.
Venmortsen doesn't approve of the final method, and the person given may come out unharmed, and the pact maker may re-awaken very unhappy with their new form, as well as whatever thing Venmortsen decided to take instead of said person.