This shield appears to be made of chitinous flesh with a jagged-toothed maw on its surface.  While wielding this shield, you can cast you can cast shield and shield of faith from it at will, requiring no concentration.

This shield is immune to game effects that would damage or destroy armor with acid such as a black pudding's Corrosive Pseudopod attack.

The shield adheres to anything that touches it or anything you hit with it. A Large or smaller creature adhered to the shield is considered grappled by you (escape DC 12). Ability checks made to escape this grapple have disadvantage and you have advantage on attack rolls against any creature grappled by the shield.  You can activate or deactivate this trait at will.  While the shield's adhesive trait is active, ability checks made to remove the shield from your grasp against your will have disadvantage. 

As a bonus action, you can perform a Shield Bash melee weapon attack with this shield using a damage die of 1d6.  If the shield's adhesive trait is active, a Large or smaller target is grappled by the shield on a hit.  While using this shield's adhesive trait to grapple a creature, you cannot make a Shield Bash attacks with it against another target.  If you make a Shield Bash attack against a Medium of smaller creature while the shield's adhesive trait is inactive, on a hit, you can force that creature to make a Strength saving throw or fall prone.  The DC for this equals 8 + your Strength modifier + your proficiency bonus.

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

Shield Mimic

Small monstrosity (shapechanger), neutral

Armor Class 17 (Natural Armor)
Hit Points 27 (6d6 + 6)
Speed 0 ft., fly 30 ft. (hover)

STR	DEX	CON	INT	WIS	CHA
15 (+2)	14 (+2)	13 (+1)	5 (-3)	13 (+1)	6 (-2)

Skills Stealth +6

Damage Immunities acid

Condition Immunities prone

Senses darkvision 60 ft., passive Perception 11

Languages Common, Undercommon

Challenge 1/4 (50 XP)

Shapechanger. The mimic can use its action to polymorph into a shield 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 Large or smaller creature adhered to the mimic is also grappled by it (escape DC 12). Ability checks made to escape this grapple have disadvantage.  The mimic can excrete, cease excreting, or dissolve its adhesive at-will.

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

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Innate Spellcasting. The mimic's innate spellcasting ability is Constitution (spell save DC 11). The mimic does not need to concentrate on concentration spells and can innately cast the following spells, requiring no material components:

At will: shield (when self or wielder is hit or targeted by magic missile), shield of faith (self only)

Mimic Shield. The mimic can be wielded as a shield and functions as an animated shield.  Wielding the mimic as a shield increases its wielder's Armor Class by 2.   If the mimic is subjected to a spell or effect that would increase it's AC, it's wielder's AC increases by that amount for the duration of the effect.  While acting as a shield for another creature the mimic takes no actions of its own.  While holding the mimic, it's wielder can use the mimic's Shield Bash attack as a bonus action, using it's own Strength and proficiency bonus for the attack roll, damage roll, and DC for the prone effect. The wielder can also 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.

While being wielded as a shield, the mimic shares its Adhesive and Grappler traits with its wielder.  Additionally, ability checks made to remove the mimic from its wielder's grasp against their will have disadvantage.

The mimic is immune to game effects that would damage or destroy armor with acid such as a black pudding's Pseudopod attack.  Non-acid game effects that would damage or destroy armor deal 1d6 damage to the mimic instead of lower its AC.  If an attack or game effect would reduce the mimic to 0 hit points while it is being wielded and its wielder has at least 1 hit point, the mimic drops to 1 hit point instead.  While the mimic has 5 hit points or less, its wielder does not benefit from the mimic's shield bonus, and loses all benefits provided by the mimic.  The mimic is considered a destroyed shield until its hit points rise above 5.

Actions

Shield Bash. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.  If the mimic is in object form, the target is subjected to one of the following effects:

• The target is subjected to the mimic's Adhesive trait.
• If the target is a Medium or smaller creature, it must succeed on a DC 12 Strength saving throw or be knocked prone.
• The mimic bites the target dealing an additional 5 (1d6 + 2) piercing damage plus 3 (1d6) acid damage.

Description

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

The mimic's innate ability to levitate allows it to function as an animated shield, protecting its bearer while also allowing them to keep their hands free.  Its ability to excrete or dissolve its adhesive at-will and ability to shapechange its maw grants a shield mimic a great deal of versatility on the battlefield, effectively able grapple foes it strikes or that strike it, bite them to deal additional damage, or shove them.

 

Shields require the Utilize action to Don or Doff. You gain the Armor Class benefit of a Shield only if you have training with it.

Notes: Damage: Bludgeoning, Damage, Control, Combat, Shapechanging, Warding, Sentient