Due to the sticky nature of your body, ability checks made to escape your grapple have disadvantage.

Your maw is a natural weapon, which you can use to make unarmed strikes. If you hit with it, you deal piercing damage equal to 1d4 + your Strength modifier, instead of the bludgeoning damage normal for an unarmed strike. Once you reach 6th level, your bite deals an additional 1d4 acid damage. When a creature is hit with this attack they must make a dc (8+STR+PROF) STR saving throw or become grappled.

Darkvision. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.

Surprise Assail. While polymorphed as an unassuming object, whenever a Large or smaller creature enters a space within 5 feet of you (or simply interacts with you), you can use your reaction to rapidly extend up to two of your limbs and make an attack against the creature. Alternatively, you can attempt to grapple them instead. You have advantage on this attack roll or grapple check if the target is unsuspecting of your identity as a mimic. Whether you are successful or not, your identity is revealed to any creatures that can see you. Additionally, if initiative hasn't been rolled yet, the target becomes surprised. You may not use your bite to grapple in this case. 

Your creature type is both humanoid and monstrosity.

You can polymorph into an inanimate object of your medium or small size that you have seen, or back into your true form. While you remain motionless, as an object you are indistinguishable from an ordinary object. If the object is expected to move and does not rouse reasonable suspicion from NPCs (i.e. a set of fully enclosed plate mail that is indistinguishable from a person in full plate mail), you are also completely indistinguishable from that object. When shapeshifting, equipment may be dropped, contained within you (as long as your shape has the dimensions to accommodate it). If you die, you revert to your true form. You retain all of your normal statistics while in this form.

You can speak, read, and understand Common, Mimic, and Under common.

Stat Improvements

While a creature is grappled buy the player they may utilize an attack action to swallow the grappled creature. The target gains the blinded and restrained conditions while swallowed. The escape dc for being swallowed is the same as to break free from the grapple (8 + STR + PROF). 

On the start of each of the player's turns the swallowed creature will take 3d4 acid damage. At 5th level 4d4. At 11th level 5d4. At 17th level 6d4.

If the creature dies while swallowed their body is consumed by the mimic and the mimic gains temp hp equal to their CON modifier + PROF. the head of the killed creature replaces any previous head in the head of the armor. after 1d6+2 days the heads begin to rot and stink. Any observer may make an insight challenge against the mimic's stealth (regardless of armor disadvantage) to notice the rotting flesh. 

Once per long rest, the user may dawn their armor to equip a new set. This process takes 1 hour and when attacked in this state all hit attacks are considered critical hits, due to the monstrosity's vulnerable state.