This weapon appears organic, resembling something made of flesh and bone.  While wielding this weapon, you can use a bonus action to polymorph it into any non-magical weapon.  This trait isn't itself magical. 

If you polymorph it into a melee weapon, it ignores the heavy property, and can be wielded normally by Small creatures.  If you polymorph it into a ranged weapon, it ignores the ammunition property, producing its own ammunition.  If you polymorph it into a weapon with the thrown property, it is considered a melee weapon for the purposes of this weapon's traits, even if used make a ranged weapon attack.

This weapon is immune to game effects that would damage or destroy weapons with acid such as a black pudding's Corrosive Form trait.

While holding this weapon, you can cast the true strike spell from it at will.  You can also cast the elemental weapon spell from it at will as a 5th-level spell to grant it a +2 bonus to attack rolls, dealing an extra 2d4 acid damage on a hit for the duration, requiring no concentration.

The weapon and the projectiles it creates produce adhesive, and adhere to anything they hit.  You can activate or deactivate this adhesive trait at will. 

While the weapon's adhesive trait is active, ability checks made to disarm you of this weapon have disadvantage.  When you hit a Large or smaller creature with a melee attack while the weapon's adhesive trait is active, that creature is considered grappled by you (escape DC 12).  Ability checks made to escape this grapple have disadvantage.  You have advantage on attack rolls against any creature grappled by this weapon.  While using this weapon to grapple a creature, you cannot make melee weapon attacks with it against another target. 

When you hit a Medium or smaller creature with a ranged weapon attack using a projectile produced and fired by this weapon while the weapon's adhesive trait is active, that creature is restrained (escape DC 12).  Ability checks made to escape this restraint have disadvantage.  Projectiles created by the weapon turn to adhesive goo on impact and cannot be retrieved.

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

Weapon 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 a bonus action to polymorph into any weapon or back into its true, amorphous form. If the mimic polymorphs into a melee weapon, it ignores the heavy property.  If the mimic polymorphs into a ranged weapon, it ignores the ammunition property.  If the mimic polymorphs into a weapon with the thrown property, it is considered a melee weapon, even if used make a ranged weapon attack. Its statistics, other than the damage and range of its weapon attacks, 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.

Adhesive Shot (Object Form Only). The mimic's projectiles adhere to anything they hit. A Medium or smaller creature adhered to by one of the mimic's projectiles is also restrained by it (escape DC 12). Ability checks made to escape the restraint have disadvantage.

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

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

Innate Spellcasting. The mimic's innate spellcasting ability is Strength (spell save DC 12). The mimic does not need to concentrate on concentration spells and can innately cast the following spells, requiring no material components:

At will: elemental weapon (self only, 5th-level spell, +2 to attack rolls, 2d4 acid damage), true strike

Mimic Weapon. The mimic can be wielded as a weapon, and functions as whatever weapon it polymorphs into.  While acting as a weapon for another creature the mimic takes no actions of its own.  An attuned wielder can command the mimic to polymorph into a different weapon as a bonus action.  The wielder can use an bonus action to command the mimic act as a dancing sword, regardless of what type of weapon the mimic is polymorphed into. 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.

When an attack using the mimic as a weapon hits, the target is subjected to the mimic's Adhesive trait if the mimic is polymorphed into a melee weapon, or its Adhesive Shot trait if it is polymorphed into a ranged weapon. Additionally, ability checks made to disarm the mimic's wielder by removing the mimic from their grasp have disadvantage. While being wielded as a weapon, 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

Weapon Attack. Melee or Ranged Weapon Attack: +4 to hit, reach or range of the weapon the mimic has polymorphed into, one target. Hit: The weapon damage die of the weapon the mimic has polymorphed into + 2.  If the mimic is polymorphed into a melee weapon the target is subjected to its Adhesive trait.  If the mimic is polymorphed into a ranged weapon the target is subjected to its Adhesive Shot trait.

Description

Weapon mimics pose as well made or even magical weapons to make themselves desirable to adventurers. They seek out caches of treasure or armories to hide among valuable objects or other weapons.  Once picked up by an adventurer, a weapon 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 weapon mimic will often try to negotiate; offering to serve the adventurer as its weapon in exchange for regular payments of food.

As an amorphous living thing, a mimic is capable of adjusting its center of mass without impacting its performance, allowing its to be wielded effectively with ease by virtually anyone.  They are even able to replicate ranged weaponry by forming and firing projectiles from their own flesh; each one a tiny simulacrum of the original which upon hitting a target, reverts to an amorphous form and grapples it.

 

Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage, Control, Buff, Combat, Shapechanging, Sentient, Versatile