Conductor of Carnage

Immutable Form. The conductor is immune to any spell or effect that would alter its form.

Painful Resistance (3/Day). If the conductor fails a saving throw, it can choose to succeed instead. If it does so, it immediately gains two gaping wounds from its Bloody Blade action. The conductor automatically fails saving throws to remove these wounds, which remain until it is reduced to 0 hit points.

Crimson Ascension (Mythic Trait; Recharges after a Short or Long Rest). If the conductor is reduced to 0 hit points, it doesn't die or gain the Unconscious condition. Instead, its hit point total resets to 216, and it immediately recharges its Curse of Carnage. For the next hour, it can use the options in the "Mythic Actions" section. Additionally, the conductor's Bloody Blade attack extends and slices through creatures it hits. When the conductor attacks with its Bloody Blade while this trait is active, compare the attack roll's total to the AC of each creature in a 15 ft. cone from it, resolving the attacks separately.

Multiattack. The conductor makes four Bloody Blade attacks. It can replace two of these attacks with a use of its Curse of Carnage, if available.

Bloody Blade. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 17 (2d8+8) slashing damage and if the target is not a Construct or Undead, it gets a gaping wound. At the end of a wounded creature's turn, it takes 5 (1d10) necrotic damage for each gaping wound it has, and must make a DC 22 Constitution saving throw, removing all gaping wounds on itself on a success. A Greater Restoration or Regenerate spell, or similar magic also closes any gaping wounds instantly.

Curse of Carnage (Recharge 5-6). Each conscious creature within 120 ft. of the conductor must succeed on a DC 22 Wisdom saving throw, taking 44 (8d10) psychic damage on a failure, or half as much damage on a success. A creature that fails its saving throw becomes cursed - while cursed in this way, it has the Frightened condition, and the creature takes 22 (4d10) psychic damage at the end of its turn unless it has dealt any amount of damage to another creature during its turn. 

Mark of Sleep. The conductor hexes one creature it can see with a sleeping hex. The target has the Unconscious condition at the start of its next turn, and experiences terrible nightmares. The creature can try to resist the nightmares to weaken the conductor - if it does, it makes a DC 22 Wisdom or Charisma saving throw (its choice). On a success, the next time the conductor regains its spend Legendary actions, it regains 2 less actions.

An unconscious creature wakes up if it takes damage, or if someone uses an action to shake or slap it awake. Once a creature ends this condition on itself, it is immune to the conductor's Mark of Sleep until the end of the conductor's next turn.

Retaliate. In response to taking damage from a creature within 10 ft. of it, the conductor makes one Bloody Blade attack against that creature. On a hit, the target takes an additional 5 (1d10) necrotic damage for each gaping wound it has.

The conductor can take legendary actions, choosing from the options below - it has a number of actions equal to the number of creatures fighting it + 1. Only one legendary action option can be used at a time, and only at the end of another creature's turn. The conductor regains spent legendary actions at the start of its turn.

Devour Nightmare. The conductor moves up to its speed. If it ends this movement within 5 ft. of an Unconscious creature, that creature takes 32 (5d12) psychic damage, and the conductor regains a number of hit points equal to the damage taken.

Bloody Blade (Costs 2 Actions). The conductor makes one Bloody Blade attack.

Festering Wounds (Costs 3 Actions). Each creature within 120 ft. of the conductor takes 5 (1d10) necrotic damage for each gaping wound it has.

If the conductor's Crimson Ascension trait has activated within the last hour, it can use the options below as legendary actions.

Expunge (Costs 3 Actions). The conductor chooses one creature within 60 ft. of it. If the target has 5 or more gaping wounds, it dies.

Vengeful Stance (Costs 2 Actions). The conductor prepares to strike at the first creature that provokes it. The conductor enters a stance, which lasts until the start of its next turn, it has the Incapacitated condition, or until it uses another legendary action. When a creature within 120 ft. of the conductor deals damage to it, the conductor teleports to an unoccupied space next to that creature, and can use its Retaliate reaction against that target.