False Appearance. While the armor remains motionless, it is indistinguishable from a normal suit of armor.

Magic Resistance. The helmed horror has advantage on saving throws against spells and other magical effects.

Spell Immunity. The helmed horror is immune to three spells chosen by its creator. Typical immunities include fireball, heat metal, and lightning bolt.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it and that creature has disadvantage on attack rolls against the mimic. In addition, movement is not restricted when grappling. 

Multiattack. The armor makes two melee attacks, only one of which can be a bite attack.

Longsword. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 6) slashing damage, or 22 (3d10 + 6) slashing damage if used with two hands.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 19 (3d8 + 6) piercing damage plus 13 (3d8) acid damage. The monster bites one creature that it has grappled already.

Shield Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) piercing damage plus 4 (1d8) acid damage and the target is now grappled (escape DC 18). Only one target can be grappled at a time.

Release. If a creature is already grappled by shield bite, the monster can repeat the damage without making an attack roll. The shied can be release as part of the same bonus action and the target remains grappled.

Reattach. The monster can reattach itself with its shield.