Clayshifter

Amorphous. The clayshifter can move through a space as narrow as 1 inch wide without squeezing.

False Appearance. While the clayshifter remains motionless, it is indistinguishable from an oily pool or wet rock.

Pseudopod. Melee Weapon Attack: +14 to hit, reach 5 ft., any number of targets. Hit: 14 (4d6 + 1) bludgeoning damage plus 25 (8d6) acid damage, and if the target is wearing nonmagical metal armor, its armor is partly corroded and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.

Weapon Replication. Melee Weapon Attack: +14 to hit, reach 10 ft.,  Hit: Replicate any nonmagical weapon attack damage, plus 25 (8d6) acid damage.

Body Spike. The clayshifter forms a spike from its arm and attempts to impale a creature. The creature must succeed on a Dexterity saving throw or take 61(12d10) piercing damage plus 49(16d6) acid damage. A creature immune to critical hits takes half damage from this.