This just gave me the idea of an item that is secretly highly radioactive. Carrying it and being around it it would slowly give the character increasingly acute symptoms of radiation poisoning.The characters can’t possibly understand what radiation poisoning is so they would think it is some sort of magical curse. However no magic users would be able figure out what it is and how to stop it.

I’m wondering how long it would take for the players themselves to figure out what it really is, or if they ever would.