Symbol of Orcus

A roughly carved ring of pitch-black demon bone. The inside of the band is adorned with Abyssal lettering, and a sigil in the shape of a ram's skull which glows faintly with a sickly green light.

A living creature that attempts to attune to this ring must make a DC15 Con save, taking 4d6 necrotic damage on a failure, or half on a success. Their maximum hit points are reduced by that amount, and cannot be recovered while still attuned to the ring.
At the end of each day (marked at the beginning of a long rest) a creature attuned to this ring must make a DC13 Con save, taking 2d6 necrotic damage and reducing their maximum hit points by the same amount on a failure.

Sentience
The Symbol of Orcus is a sentient, chaotic evil item with an Intelligence of 14, a Wisdom of 10, and a Charisma of 14. It has hearing and darkvision out to a range of 120 feet.
Personality
The Symbol's purpose is to bend its wearer and others to Orcus' will. The band is cold, cruel, nihilistic, and bereft of humor. It feigns allegience to its current wearer, and makes grandiose promises of power in exchange for the souls of the living. Removing the ring requires a DC15 Wisdom check. The DC of this check increases by 1 for every soul absorbed by the ring.
The ring grants the following benefits while worn, once the souls of living sentient creatures have been fed to it:

0 Souls

Darksight
Hearing and darkvision out to 120 feet.
Scion of Orcus
If you are slain while wearing this ring, the energy you have absorbed fills your dead body with unlife, and you rise again as a powerful but mindless zombie. The power gained is proportional to the amount of souls fed to the ring.
Aura of Undeath
Any effect that senses a creature's type will see the wearer of this ring as Undead.
Undead summoned by followers of Orcus will not view the wearer as a threat unless directly attacked.
Grants access to the Black Tower.

1 Soul

Undead Endurance
Your Constitution score increases by 2, to a maximum of 20.

3 Souls

End of day necrotic damage increased to 3d6.
Negative Protection
You gain a +2 bonus to AC.

6 souls

End of day necrotic damage increased to 6d6.
Undead Strength
Your Strength score increases by 2, to a maximum of 20.
Negative Aura
Waves of negative energy emanate from the ring, surrounding you. You gain resistance to Necrotic damage (except that of the ring itself) and any creature that hits you with a melee attack takes 2d8 necrotic damage.