This fearsome steel helm's visor is lined with teeth.  While wearing this helm, you gain the following benefits:

• You have darkvision out to a range of 60 feet.
• You can use an action to cast the following spells from the helm at will, requiring no concentration (spell save DC 15): acid splash (1d6 acid damage), eyebite, and mind blank.  The spells end if the hem is removed.
• You can breathe normally in any environment, and you have advantage on saving throws made against harmful gases and vapors (such as cloudkill and stinking cloud effects, inhaled poisons, and the breath weapons of some dragons).
• When you make a Wisdom check, you can replace the number you roll with a 15.
• As a bonus action, you can make a Headbutt melee weapon attack against one creature grappled by you.  A Headbutt attack made with this helm is an Unarmed Strike that deals an additional 1d4 + 1 piercing damage plus 1d4 acid damage.
• Checks made to remove the helm from your head against your will have disadvantage.

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

Helm Mimic

Tiny monstrosity (shapechanger), neutral

Armor Class 12 (Natural Armor)
Hit Points 5 (2d4)
Speed 5 ft.

Skills Stealth +5

Damage Immunities acid

Condition Immunities prone

Senses darkvision 60 ft., passive Perception 15

Languages Common, Undercommon

Challenge 1/4 (50 XP)

Shapechanger. The mimic can use its action to polymorph into a helmet 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.

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

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

Innate Spellcasting. The mimic's innate spellcasting ability is Wisdom (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: acid splash (1d6 acid damage), eyebite (self or wearer), mind blank (self or wearer only)

Mimic Helm. The mimic can be worn as headwear, and functions as a necklace of adaptation while being worn.  While acting as headwear for another creature the mimic takes no actions of its own.  An attuned wearer can sense what the mimic senses as long as it it wearing the mimic. 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.

While wearing the mimic, whenever the wearer makes a Wisdom check, it can replace the number it rolls with a 15.  The wearer can also use a bonus action to use the mimic's Headbutt as an Unarmed Strike against a creature it has grappled, using its own Strength modifier and proficiency bonus for the attack roll and bludgeoning damage.  Additionally, checks made to forcibly remove the mimic from its wearer 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 Corrosive Form trait.  Non-acid game effects that would damage or destroy items deal 1d6 damage to the mimic instead of lower its damage rolls.  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 or less the wearer loses all benefits provided by the mimic.  The mimic is considered a destroyed item 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.  If the target is wearing the mimic, it takes double damage.

Headbutt. Melee Weapon Attack: +3 to hit, reach 5 ft., one target grappled by the mimic that isn't wearing the mimic. Hit: 2 (1 + 1) bludgeoning damage plus the mimic's Bite damage.

Smother. Melee Weapon Attack: +5 to hit, reach 0 ft., one creature wearing the mimic. Hit: If the mimic is in object form, the target is subjected to its Adhesive trait.  Until it escapes, the target is blinded and at risk of suffocating. In addition, at the start of each of the target's turns, the target takes 3 (1d4 + 1) bludgeoning damage.

Description

Helm mimics pose as well made or even magical helmets 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 helm 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 helm mimic will often try to negotiate; offering to serve the adventurer as its headwear in exchange for regular payments of food.

Rather that wait for their bite to expose an enemy to their acidic saliva, helm mimics expectorate with surprising accuracy.  They are capable of shapechanging their respiratory system, drastically altering it to enable them to breathe in any environment.  They can also alter their "eye holes" through a combination of shapechanging and bioluminescence to emulate the the glowing glare of some dread power.  Due to their nature as headwear, they shield their wearers from psychic damage and attempts at intrusion into their minds.  Finally, they are incredibly perceptive, and will share their insights with a creature they have bonded with.

Notes: Damage: Bludgeoning, Buff, Debuff, Detection, Shapechanging, Warding, Headwear, Sentient