While wearing this ring, you can use an action to cast the true polymorph spell from it at will to polymorph yourself into a mimic.  Ability checks made to remove the ring from your hand against your will have disadvantage.

The ring has a 50% chance of actually being a mimic.  Casting the identify spell on the ring reveals if it is a mimic or not.  If the ring is revealed to be a mimic, how the mimic reacts to being discovered is up to the DM.

Ring Mimic

Tiny monstrosity (shapechanger), neutral

Armor Class 11
Hit Points 5 (2d4)
Speed 5 ft.

Skills Stealth +5

Damage Immunities acid

Condition Immunities prone

Senses darkvision 60 ft., passive Perception 11

Languages Common, Undercommon

Challenge 1/8 (25 XP)

Shapechanger. The mimic can use its action to polymorph into a ring or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A Medium or smaller creature adhered to the mimic is also grappled by it (escape DC 11). Ability checks made to escape this grapple have disadvantage.  The mimic can excrete, cease excreting, or dissolve its adhesive at-will.

Damage Transfer. While adhering to or worn by a creature, the mimic takes only half the damage dealt to it (rounded down), and that creature takes the other half.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary ring.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it and a creature grappled by the mimic has disadvantage on attack rolls against the mimic.

Innate Spellcasting. The mimic's innate spellcasting ability is Constitution (spell save DC 10). The mimic does not need to concentrate on concentration spells and can innately cast the following spells, requiring no material components:

At will: true polymorph (polymorphs attuned wearer into a mimic)

Mimic Ring. The mimic can be worn as a ring.  While acting as a ring for another creature the mimic takes no actions of its own.  An attuned wearer can telepathically command the mimic to cast a spell from the mimic's Innate Spellcasting trait, requiring the spell’s normal casting time and using the mimic's spellcasting DC.  Ability checks made to remove the mimic from its wearer's hand against their will have disadvantage.

The mimic is immune to game effects that would damage or destroy items with acid such as a black pudding's Pseudopod attack.  Non-acid game effects that would damage or destroy items deal 1d4 damage to the mimic.  If an attack or game effect would reduce the mimic to 0 hit points while it is being worn and its wearer has at least 1 hit point, the mimic drops to 1 hit point instead.  While the mimic has 1 hit point, its wearer loses all benefits provided by the mimic.  The mimic is considered a destroyed ring until its hit points rise above 1.

Actions

Pseudopod. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) piercing damage plus 2 (1d4) acid damage.

Acid Coat. As a bonus action, mimic excretes acid from its skin.  For 1 minute, any creature that touches the mimic or hits it with a melee attack while within 5 feet of it takes 4 (1d8) acid damage. Any nonmagical weapon made of metal or wood that hits the mimic corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed.  Nonmagical ammunition made of metal or wood that hits the mimic is destroyed after dealing damage.

In addition, when the mimic hits a target with a melee weapon attack, nonmagical armor worn by the target is partly dissolved and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.

Description

Ring mimics pose as beautifully crafted, magical rings to make themselves desirable to adventurers. They seek out caches of treasure to hide among valuable objects.  Once picked up by an adventurer, a ring mimic will travel with its new owner.  By night, while the owner sleeps, the mimic will often rummage through their belongings in search of food.  If discovered (usually when a new owner attempts to identify it) a ring mimic will either attack or try to negotiate; offering to continue to serve as a magic ring for the adventurer in exchange for regular payments of food.

Ring mimics possess the power to temporarily transform creatures they bond with into members of their own kind, granting those creatures a mimic's impressive shapechanging abilities.  As mimics can flawlessly imitate any object, this ability to become one can prove invaluable to any creature with a need for stealth and to evade notice.

Notes: Buff, Shapechanging, Jewelry, Sentient