Reduced Threat. A reduced-threat monster takes a −2 penalty on attack rolls (included in the stat block), ability checks (included in the stat block for skill proficiencies), saving throws (included in the stat block for saving throw proficiencies), and saving throw DCs (included in the stat block).
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.
Multiattack. The helmed horror makes two longsword attacks.
Longsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit:8 (1d8 + 4) slashing damage, or 9 (1d10 + 4) slashing damage if used with two hands.