This gold ring bears a symbol of binding on it, and functions identically to a ring of spell storing in addition to its other properties.

Curse. This ring is cursed, and becoming attuned to it extends the curse to you. As long as you remain cursed, you are unwilling to part with the ring, keeping it on your finger at all times. Additionally, you must make a DC 10 Charisma saving throw during each long rest. On a success, the rest proceeds as normal, but on a failure the rest is disturbed by strange Abyssal chants that intrude upon your mind and you do not recover any hit dice during the long rest.

When this ring is first found, it contains a bound demon with challenge rating 5 or lower, as determined by the DM. This demon takes up a number of stored spell levels equal to the half the challenge rating of the demon, rounded down. An identify spell reveals the item as a normal ring of spell storing and that spell levels are being stored but it cannot identify that they are being used to store a captured demon. Attempting to cast the "spell" being stored results in nothing happening.

The first time you use this ring to cast a spell of 3rd level or higher you must make a DC 15 Charisma saving throw. On a success, the spell is cast as normal. On a failure, the spell is lost and the stored demon is unleashed and is hostile towards you. Defeating the demon banishes it back to the Abyss. A banishment spell cast on the ring will also rid it of the stored demon.

Once the ring has been cleansed of the demon, you may continue to use it as a normal ring of spell storing with the additional property that if you cast the spell summon greater demon using the additional "circle of blood" rites described at the end of the spell, you can store the spell in the ring using half the number of slots, rounded down.

Notes: non-good wizard or warlock, Buff, Utility, Cursed, Jewelry