This exquisite instrument has an artistically carved monster's face upon its surface.  A creature that attempts to play the instrument without being attuned to it must succeed on a DC 15 Wisdom saving throw or take 2d4 psychic damage.

While holding this instrument, you can use an action to speak the instrument's command word, causing it to polymorph into any Medium-sized or smaller instrument.  Additionally, the instrument can be used as a melee weapon, functioning as a mace.

The instrument produces adhesive, and adhere to anything it touches.  You can activate or deactivate this adhesive trait at will. 

While its adhesive trait is active, ability checks made to disarm you of this instrument have disadvantage.  When you hit a Large or smaller creature with a melee attack with this instrument its adhesive trait is active, that creature is considered grappled by you (escape DC 11).  Ability checks made to escape this grapple have disadvantage.  You have advantage on attack rolls against any creature grappled by this instrument.  While using this instrument to grapple a creature, you can't make melee weapon attacks with it against another target, can't cast spells from it, and can't use it as a spellcasting focus. 

You can use an action to play the instrument and cast one of its spells: Fly, Invisibility, Levitate, Protection from Evil and Good, Alter Self, Disguise Self, True Polymorph (self only). Once the instrument has been used to cast a spell, it can’t be used to cast that spell again until the next dawn. The spells use your spellcasting ability and spell save DC.

You can play the instrument while casting a spell that causes any of its targets to be charmed on a failed saving throw, thereby imposing disadvantage on the save. This effect applies only if the spell has a somatic or a material component.

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

Instrument Mimic

Small monstrosity (shapechanger), neutral

Armor Class 12 (Natural Armor)
Hit Points 21 (6d6 + 0)
Speed 10 ft.

Skills Performance +4, Stealth +6

Damage Immunities acid

Condition Immunities deafened, 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 bard's instrument 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 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 musical instrument.

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

Innate Spellcasting. The mimic's innate spellcasting ability is Charisma (spell save DC 12). The mimic can innately cast the following spells, requiring no material components:

1/day each: fly, invisibility, levitate, protection from evil and good, alter self, disguise self, true polymorph (creature into object only)

Spellcasting. The mimic is a 5th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 12, +4 to hit with spell attacks). It has the following bard spells prepared:

Cantrips (at will): friends, minor illusion, vicious mockery

1st level (4 slots): charm person, dissonant whispers, sleep, thunderwave

2nd level (3 slots): calm emotions, enthrall, hold person

3rd level (2 slots): fear

Mimic Instrument. The mimic can be wielded as an instrument, and functions as an instrument of the bards.  While acting as an instrument of the bards for another creature the mimic takes no actions of its own.  An attuned wielder can command the mimic to polymorph into a different instrument as an action.  While holding the mimic, an attuned wielder shares the mimic's Innate Spellcasting trait, using its own spellcasting ability and spell save DC in place of the mimic's.  Additionally, an attuned wielder can command the mimic to cast a spell from the mimic's Spellcasting trait, requiring the spell’s normal casting time, using the mimic's spellcasting DC, and using the mimic's spell slots.

While polymorphed into an instrument, the mimic also functions as a mace.  When a melee attack using the mimic as a weapon hits while the mimic is polymorphed into an instrument, the target is subjected to the mimic's Adhesive trait.  While a creature is subject to the mimic's Adhesive trait, it cannot be used to make another weapon attack, can't have spells cast from it, and can't be used as a spellcasting focus.  Ability checks made to disarm the mimic's wielder by removing the mimic from their grasp have disadvantage. While being held by an attuned wielder, the mimic shares its Grappler trait with its wielder against any creature subjected to its Adhesive trait.

The mimic is immune to game effects that would damage or destroy weapons with acid such as a black pudding's Corrosive Form trait.  Non-acid game effects that would damage or destroy weapons 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 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, it cannot be used to make weapon attacks, and the wielder loses all benefits provided by the mimic.  The mimic is considered a destroyed weapon until its hit points rise above 5.

Actions

Pseudopod. 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 its Adhesive trait.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage plus 3 (1d6) acid damage.

Description

An instrument mimic poses as a musical instrument, most often as an instrument of the bards. They seek out caches of treasure or music halls to hide among valuable objects or other instruments.  Once picked up by an adventurer, an instrument mimic will travel with its new owner.  By night, while the owner sleeps, the mimic will rummage through their belongings in search of food or sheet music to read and learn.  If discovered (usually when a new owner attempts to identify it) an instrument mimic will often try to negotiate; offering to serve the adventurer with its bardic power in exchange for regular payments of food.

These mimics imitate musical instruments not only in appearance, but also in function, and can produce beautiful music.  They are an intelligent variety of mimic with a great appreciation for music.  Like the magical instruments they imitate, they are very particular about who they will allow to play them, attacking a would-be wielder they do not approve of with vicious mockery.  Also like the magic instruments they pose as, they grant a wielder they have bonded with the use of their own spells.

Notes: bard, Damage, Control, Buff, Utility, Combat, Shapechanging, Instrument, Sentient