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

Corrode Metal. Any nonmagical weapon made of metal that hits the horror corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the ooze is destroyed after dealing damage.

The horror can eat through 2-inch-thick, nonmagical metal in 1 round.

Multiattack. The horror can make 1 attack with acid breath, or 2 pseudopod attacks.

Acid Breath. (Recharge 5–6). The horror exhales acid in a 15-­foot line that is 5 feet wide. Each creature in that line must make a DC 11 Dexterity saving throw, taking 22 (5d8) acid damage on a failed save, or half as much damage on a successful one. If a creature who failed their save is wearing nonmagical metal armor, its armor is partly corroded and takes a permanent adlnd cumulative -1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.

Pseudopod. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit:  15 (2d6 + 8) bludgeoning damage plus 14 (4d6) acid damage. 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. 

Disarm. If the horror is hit by an attack that uses a melee weapon that it can see, it can use its reaction to attempt to disarm the attacker. The attacker must succeed on a DC 13 Strength saving throw or have its weapon torn from its grasp. If the weapon is nonmagical and made of metal, the weapon is absorbed by the horror and the horror regains hit points equal to double the damage dealt. 

The adamantine horror can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The adamantine horror regains spent legendary actions at the start of its turn.

Attack. The horror makes 1 pseudopod attack.

Move. The adamantine horror moves up to his speed without provoking opportunity attacks.