Why do wild mages always have the worst ideas? The first mimic of this sort came from such a spell-caster who charmed a mimic, twisted it to a staff, and imbued it with chaotic power until it became magical in its own right. The mage though it focused the surge into just duplicating spells, but the mimic was feeding off of him. One thing that can't be charmed is a mimic's hunger and eventually, well, it drained the mage's last bit of lifeforce to make an easy meal. lt has found many a power-hungry mage just as gullible since then.

Staff of the Mimic

Medium monstrosity (shapechanger), neutral

Armor Class 11 (natural armor)

Hit Points 78 (12d8 + 24)

Speed 15 ft

Str 15 Dex 10 Con 14 Int 12 Wis 13 Cha 16

Skills Arcana+3, Stealth +4

Damage Immunities acid; damage from spells a creature holding it casts

Condition Immunities prone

Sensesdarkvision 60 ft, passive Perception 11

Languages -

Challenge 4 (1,100 XP)

Shapechanger.

The mimic can use its action to polymorph into a +2 magic quarterstaff 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 Huge 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 activate and deactivate this ability as a bonus action.

Archmimic.

If the mimic reduces a spell-caster to 0 Hit Dice, that creature falls to 0 hit points and begins dying. The mimic heals a number of Hit Points equal to the total level of spells the creature can still cast. If the mimic kills a spell caster with a physical attack, it gains Advantage on all attacks for a number of rounds equal to that creature's normal hit dice.

False Appearance (Object Form Only).

While the mimic remains motionless, it is indistinguishable from an ordinary +2 magic quarterstaff.

Grappler.

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

Magic Weapons.

The mimic's weapon attacks are magical.

Actions

Pseudopod.

Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite.

Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage plus 4 (1d8) acid damage.

Reactions

Spell Mimicry

The mimic casts a spell that a creature holding it just cast at the same or a different target, as long as the spell is 3rd level or less. The mimic's spellcasting stat is Charisma. (+5 to hit with spell attacks, DC 13) However, the creature holding it also loses Hit Dice equal to the level of the spell. Usually, the mimic uses this Reaction as directed by the creature holding it using a Bonus Action to do so. However, the mimic can do this of its own accord whether the creature holding it wants it too or not.