Ancient Dullahan

Baleful Glare. When a creature that can see the eyes of the dullahan’s severed head starts its turn within 30 feet of the dullahan, the dullahan can force it to make a DC 21 Wisdom saving throw if the dullahan isn’t incapacitated and can see the creature. On a failed save, the creature is frightened until the start of its next turn. While frightened in this way, the creature must take the Dash action and move away from the dullahan by the fastest available route, regardless of the danger. A doomed creature that fails this saving throw is restrained while frightened instead. A creature that fails the save by 5 or more with less than half its maximum hit points dies immediately. Creatures summoned by the dullahan are unaffected by this trait.

Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can’t see the dullahan until the start of its next turn, when it can avert its eyes again. If the creature looks at the dullahan in the meantime, it must immediately make the save.

Cumulating Critical. When the dullahan hits a creature with an attack that isn't a critical hit, its critical hit range increases by 1 until it hits a creature with a critical hit (EX: 20, 20 to 19, 20-18, etc.).

Decapitated. Critical hits dealt against the Dullahan are treated as normal hits. The Dullahan cannot be decapitated. The head almost always sticks close to the body but if it departs (flying speed (hover) 30 ft.), it is a small creature and has an AC of 15 and 40 hit points, otherwise using the statistics of a Death Tyrant. The head and body use the same initiative, but can act either before or after the body (dullahan's choice). If it is reduced to zero, it vanishes and returns to the side of the body at the end of The Dullahan's turn on the next round.

Headless Helpers. When the dullahan kills a humanoid creature by decapitating it, the creature rises at the start of its next turn as a Dullahan (KP) under the Ancient Dullahan's control. It obeys the dullahan's commands to the best of its ability, even if they don't share a language.

Headless Summoning (Recharges after a Short or Long Rest). If the dullahan is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 146 hit points. In addition, it summons 5 (1d4 + 3) death’s heads, at least one of each type, in unoccupied spaces within 5 feet of it. The death’s heads are under the dullahan’s control, act immediately after the dullahan in the initiative order, and the save DC's for each death's head attack is 17. Additionally, the dullahan can now use the options in the “Mythic Actions” section. Award a party an additional 75,000 XP (150,000 XP total) for defeating the dullahan after it uses Headless Summoning.

Legendary Resistance (3/Day). If the dullahan fails a saving throw, it can choose to succeed instead.

Magic Resistance. The dullahan has advantage on all saves against spells or other magic abilities.

Master of the Blightscale. The dullahan is always mounted upon a Young Blightscale Dragon when entering combat. The dragon gains 15 temporary hit points at the start of each of the dullahan's turns. It can also use its reaction to make a melee attack against a target the dullahan attacks.

Mounted Combatant. The dullahan has advantage on any ability check or saving throw to remain mounted. When targeted by an attacked, the dullahan can force that attack to target its steed instead. If its mount is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails. If the dullahan falls from its mount, it always lands on its feet. Also, while mounted, it has advantage on attack rolls made against unmounted creatures smaller than its nightmare.

Relentless Advance. Moving through difficult terrain doesn’t cost the dullahan extra movement, and the dullahan can move across the surface of water as if it were harmless, solid ground.

Unusual Nature. The dullahan doesn’t require air, food, drink, or sleep.

Multiattack. The dullahan makes three attacks. It can replace one attack with a use of Seal the Doom.

Battleaxe. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) slashing damage, or 11 (1d10 + 6) slashing damage if used with two hands, plus 11 (2d10) necrotic damage. If the dullahan scores a critical hit against a creature, the target must succeed on a DC 20 Constitution saving throw or the dullahan cuts off the target’s head. The target dies if it can’t survive without the lost head. A creature that doesn’t have or need a head, or has legendary actions, instead takes an extra 27 (6d8) slashing damage.

Spine Whip. Melee Weapon Attack: +14 to hit, reach 40 ft., one target. Hit: 13 (2d6 + 6) slashing damage plus 10 (3d6) necrotic damage. If the target is a creature, it must succeed on a DC 21 Constitution saving throw or be dragged to an unoccupied space within 15 ft. of the dullahan and fall prone as it is wracked with pain.

Necrotic Flame Skull. Ranged Spell Attack: +12 to hit, range 120 ft., one target. Hit: 15 (2d10 + 4) fire damage plus 22 (4d8 + 4) necrotic damage. The target must succeed on a DC 20 Charisma saving throw or be enraged until the end of its next turn. While enraged, the target has advantage on melee attack rolls and must move to and attack the nearest creature other than the dullahan. In addition, attack rolls against the enraged target have advantage.

Hunting Hounds. The dullahan summons 2d4 Harrow Hounds, Hell Hounds, Hounds of the Night, or Lich Hound to assist it. The hounds appear at the end of the dullahan's next turn, rolling for initiative as a group.

Seal the Doom. The dullahan points at a creature doomed by Deathly Doom within 40 feet of it that it can see. The creature must make a DC 21 Constitution saving throw. On a failure, the target has its current hit points halved, or immediately drops to 0 hit points if it is below half its hp maximum. On a success, the target is immune to the dullahan’s Seal the Doom for the next 24 hours.

Deathly Doom. The dullahan magically dooms a creature for 1 hour. It can have up to two creatures doomed at a time. If it dooms a third, the effect on the first previous target ends. The dullahan knows the direction to the doomed creature as long as both are on the same plane of existence.

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

Attack. The dullahan makes one attack.

Frightful Presence (Costs 2 Actions). Each creature of the dullahan’s choice within 30 feet of it must succeed on a DC 20 Wisdom saving throw or become frightened of the dullahan until the end of its next turn. If a creature failed the save by 5 or more, a Beheaded Vengeful Spirit will appear, forced to serve the dullahan that took its life. The spirit shares initiative with the dullahan, but acts immediately before it, fighting until its defeat or the dullahan is slain. On a success, a gains advantage on its next saving throw against the dullahan's baleful glare.

Head Hunt (Costs 3 Actions). The dullahan moves up to its speed without provoking opportunity attacks and makes one Battleaxe attack with advantage. If the attack hits, but is not a critical hit, the attack deals an extra 27 (6d8) necrotic damage. 

If the dullahan’s Headless Summoning trait is active, it can use the options below as legendary actions.

Coordinated Assault. The dullahan makes a Battleaxe attack, and then each death’s head the dullahan can see within 30 feet of the target can use its reaction to move towards the target and make a melee attack.

Headless Wail (Costs 2 Actions). An echoing shriek issues from the dullahan’s headless stump. Each creature of the dullahan’s choice within 10 feet of it must make a DC 20 Wisdom saving throw. Each creature takes 33 (6d10) psychic damage on a failed save, or half as much damage on a successful one. If one or more creatures fail the saving throw, the dullahan gains 15 temporary hit points and summons 1 death's head for each creature that failed.

Better than One (Costs 3 Actions). The dullahan targets one death's head under its control and empowers it. The empowered head then transforms into a new creature, regaining hit points to match its new form. Roll a d4 to determine what creature the head transforms into:

1. Flutterflesh
2. Vargouille Reflection
3. Flameskull
4. Beholder Zombie

In this new form, the head gains a +4 bonus to its new attack rolls and AC, and regains hit points equal to all necrotic damage it deals at the end of its turn.