This armor appears as a suit armor, and is tacky to the touch.  The armor is immune to game effects that would damage or destroy armor with acid such as a black pudding's Pseudopod attack.  While wearing this armor, you gain the following benefits:

• You are immune to acid damage.
• You adhere to anything that touches you. A Huge or smaller creature adhered to you is also grappled by you (escape DC 13). Ability checks made to escape this grapple have disadvantage.  You can activate or deactivate this trait at-will.
• You have advantage on attack rolls against any creature grappled by you.
• You gain resistance to all damage until you have taken a total of 100 damage, at which point the armor loses this trait until you complete a long rest.
• You can use your action to cause the armor to polymorph into a set of clothes, common or a suit of studded leather, half plate, or plate armor.  The armor provides its wearer with the AC of whatever type of armor it polymorphs into.  Regardless of what type of armor it is polymorphed into, the armor does not have a Strength score requirement and does not incur disadvantage on Stealth checks.
• You can spend an action to gain a +1 bonus to AC for 1 hour.  After doing so, you cannot do so again until the bonus fades.

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

Armor Mimic

Medium monstrosity (shapechanger), neutral

Armor Class 13 (16 In Half Plate Form, 18 In Plate Form)
Hit Points 105 (10d10 + 50)
Speed 20 ft.

STR	DEX	CON	INT	WIS	CHA
17 (+3)	12 (+1)	20 (+5)	10 (+0)	13 (+1)	10 (+0)

Skills Stealth +5

Damage Immunities acid

Condition Immunities prone

Senses darkvision 60 ft., passive Perception 11

Languages Common, Undercommon

Challenge 4 (1,100 XP)

Shapechanger. The mimic can use its action to polymorph into a suit of clothes, common, studded leather, half plate, or plate armor or back into its true, amorphous form. Its statistics, other than its AC, 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 Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 13). Ability checks made to escape this grapple have disadvantage.  The mimic can excrete, cease excreting, or dissolve its adhesive at-will.

Damage Transfer. While 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 a normal suit of armor.

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 15). The mimic does not need to concentrate on concentration spells and can innately cast the following spells, requiring no material components:

At will: warding bond (self only, targets mimic as though cast by wearer)

Mimic Armor.  The mimic can be worn as a suit of armor, and functions as whatever armor it polymorphs into.  Regardless of what type of armor it is polymorphed into, the mimic does not have a Strength score requirement and does not incur disadvantage on Stealth checks.  A creature wearing the mimic has total cover against effects outside the mimic.  Attacks made against the wearer are made against the mimic instead.  While acting as armor for another creature the mimic takes no actions of its own, and is no longer immune to the prone condition.  Additionally, the mimic shares its Adhesive and Grappler traits with its wearer.

The mimic's wearer can don or doff the mimic as an action. The wearer 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.

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 1d10 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 worn and its wearer has at least 1 hit point, the mimic drops to 1 hit point instead.  While the mimic has 10 hit points or less, it loses its Damage Transfer trait, its wearer takes all damage dealt to the mimic in the mimic's stead, and the wearer loses all benefits provided by the mimic.  The mimic is considered a destroyed suit of armor until its hit points rise above 10.

Actions

Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 4 (1d8) acid damage.

Swallow.  If a creature has donned the mimic as armor, the mimic can use an action to automatically swallow that creature. A swallowed creature is blinded and restrained, it has total cover against effects outside the mimic, and it takes 14 (4d6) acid damage at the start of each of the mimic's turns.  Attacks made against the target are made against the mimic instead.  When the mimic moves, the swallowed creature moves with it.

Description

This variety of mimic has learned to imitate not only the appearance of armor, but its durability and protective hardness.  Armor mimics often sneak their way into castles or armories where suits of armor are sure to be displayed or stored. 

Armor mimics allow themselves to be dismantled and donned, with each piece of armor connected to the breastplate through thin, nearly invisible strings of amorphous flesh.  Once an armor mimic has been donned, the creature wearing it is all but doomed, practically already in the mimic's stomach.

However, many armor mimics are intelligent enough to realize that feeding in this manner is a sure way to be discovered, hunted down, and killed.  Instead, they prefer to offer their formidable armored hides in service to adventurers or guards in exchange for regular payments of food.

 

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This armor consists of a fitted metal chest piece worn with supple leather. Although it leaves the legs and arms relatively unprotected, this armor provides good protection for the wearer's vital organs while leaving the wearer relatively unencumbered.

Notes: Buff, Utility, Combat, Shapechanging, Warding, Sentient