A ring of Myrkul was a relatively thick gold band painted black adorned with an image of a white flaming skull (the symbol of Myrkul).
Evil, intelligent creatures who passed within 30 feet of a ring of Myrkul were drawn to it.
Rings of Myrkul served as keys for priests of Myrkul to enter a temple of Myrkul safely. They also allowed the wearer the ability to walk among the undead without being attacked (unless the wearer attacked the undead). Additionally, the rings had a few offensive abilities, including casting the spells chill touch and ghoul touch.
When Myrkul was killed in 1358 DR, his secret temple in the Mere of Dead Men in the Sword Coast North sank into the swamp. The priests escaped into the Mere but were killed, their rings falling into the possession of the beasts of the Mere.