AO Helmed Horror

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

Energy Pool: The Horror has a pool of energy that it can expend on a variety of actions. Each round it gains one point to it's pool, in addition to any other source it may have. It has a starting pool of 3 points.

Spell Immunity. The helmed horror is immune to three spells chosen by its creator. Typical immunities include fireball, heat metal, and lightning bolt. In addition, when targeted by a spell it is immune to, it absorbs the energy, giving it temporary hit points equal to the damage of the spell

Spell Absorption: Each spell that the horror is targeted by will be partially absorbed by it, adding to a pool of energy that can be used on specific actions. The horror gains a point to it's Energy pool for each spell level that targets it. 

Multiattack. The armor 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.

Energy Release. (2 points from pool) The horror releases contained energy in a 15 foot cone. Each creature makes a DC 15 Dex save, taking 4d6 force damage on a fail, or half on a success.

Bonus Actions!

Animate Flying Sword minion: (1 point from pool) The armor transfers stored arcane energy into a nearby weapon, animating a sword and directing it to make an immediate attack.

Attack them!: The horror commands another construct to make an attack on a creature within range

Riposte: It can make an attack against a creature that just attacked it

Re-animate: (1 point from pool) Any construct that drops to 0 hp can be instantly returned to 1 hp.

Round 1:

Siphon and Empower: (1 point per empowered construct) The Helmed Horror siphons off energy, adding 2 points to its energy pool. It then imbues other constructs in the room with extra magical energy. Every empowered creature adds 1d6 damage to their attacks for 2 rounds.

Round 2:

Overcharge!: The horror channels it's arcane force into it's sword, gaining 2d6 force damage for the next round. Doing so draws too much energy from the spell forge and releases a wild magic surge. Have each player roll and select the roll that makes the most sense for the situation. Each round after, the sword may remain empowered if an additional energy point is spent on it. 

Round 3:

 Fire Fire! (1point per line of flame): the horror directs the green flames from the brazier in a 40 foot long, 5 foot wide line, 10 feet tall. Any creature within, or attempting to cross the flames must make a DC 15 Strength saving throw, taking 3d8 force damage on a fail and pushed out of the wall. On a success they take half damage and can cross the wall. Attacks involving lightweight projectiles are made at disadvantage across the barrier

 When Bloodied:

Energy Surge: (varied points from pool, trigger on bloodied) The helmed horror releases part of it's energy pool exposing nearby creatures to a blast of arcane energy. Each creature within a 15ft radius makes a DC 15 dex save, taking 2d6 force damage per point on a fail, or half on a success. Additionally, it's AC goes down by 2 due to the damage sustained by this point.