I posted this elsewhere, but I thought it would be useful to put it here. I modified an old 2e item for 5e. It is a little powerful for my level 4 PCs (level 3 when they found it), but it has some drawbacks, and ultimately it is a plot hook for some adventuring in the Mere of Dead Men...
Ring of Myrkul
The ring consists of a gold band painted black and adorned with the white skull of Myrkul. It glows faintly in the presence of a detect evil spell.
Requires attunement.
Properties:
**The wearer may Walk Among The Dead (all mindless undead and intelligent undead with 6 hit dice or less will not attack the ring wearer unless the wearer attacks them first).
**Moreover, if the wearer can make spell attacks, he or she may cast:
-Chill Touch (as per PHB p. 221) 3/day;
-Ghoul Touch 1/day --- Range: Touch; Duration: 1 minute; Effect: Imbuing you with negative energy, this spell allows you to paralyze a single living humanoid for the duration of the spell. Make a melee spell attack against a humanoid within your reach. If you hit, the target is paralyzed for the duration of the spell or until it succeeds on a Constitution saving throw. The target may attempt a saving throw once per round at the end of its turn. While paralyzed, it exudes a carrion stench that causes any living creature (except the caster) within a 25 foot cube (from the center of the target's body, aka within 2 squares) to become poisoned until it succeeds on a Constitution saving throw, which it may attempt at the end of any turn if it has moved outside the zone.
**Sufficiently evil intelligent creatures within 90 feet are drawn to the ring, with a strong desire to touch and possess it. The compulsion to touch the object can be overpowering; a DC 15 Wisdom save releases an affected creature from the compulsion. A creature that successfully saves must repeat the saving throw 1d6x10 minutes later if they are still within range.
**Wearing the ring causes one's body temperature to drop several degrees cooler, and over time one's voice becomes more like a raspy whisper unless the wearer consciously decides to speak normally.
**Before the Time of Troubles, if the ring wearer was a Myrkulite priest, he could invoke a number of other effects, but these greater necromantic powers vanished when Myrkul died.
**Some other situational powers to the ring may exist, that are tied to specific locations especially (un)holy to Myrkul, or to proximity with other Myrkulite relics.