Ever-Serving Unrest (Mythic Trait; Recharges after a Short or Long Rest). If the Lich that the Admiral serves uses a Mythic Trait, whether the Admiral is dead, unconscious, or neither, the Admiral enters a state of ascended undeath as long as his Lich is in that Mythic Form. When the Admiral enters this form, he releases a wave of dark energy, causing all creatures within 60 feet of the Admiral to make a Constitution saving throw or take 17 (5d6) necrotic damage and 17 (5d6) force damage, or half as much on a successful save.  While in this form, the Admiral is immune to all damage, but has a halved movement speed. If the Lich that the Admiral serves dies, this trait ends and the Admiral dies as well.

Action Surge (2/Day). On the Admiral's turn, you can take one additional action..

Mage Slayer. When a creature within 5 feet of the Admiral casts a spell, he can use his reaction to make a melee weapon attack against that creature. Also, when the Admiral damages a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.

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

Marshal Undead. Unless the Admiral is incapacitated, it and undead creatures of its choice within 60 feet of it have advantage on saving throws against features that turn undead.

Sentinel. When the Admiral hits a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn. The Admiral ignores the disengage action when making opportunity attacks. Also, when a creature within 5 feet of the Admiral makes an attack against a target other than the Admiral, he can use his reaction to make a melee weapon attack against the attacking creature.

Innate Spellcasting. The Admiral's spellcasting ability is Charisma (spell save DC 23). The Admiral can innately cast the following spells, requiring no material components:

At Will: contagion, darkness, gift of alacrity, hellish rebuke (5th level), life transference, misty step, thunder step

Spellcasting. The Admiral is a 19th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 23, +19 to hit with spell attacks). It has the following  spells prepared:

1st level (4 slots): command, compelled duel, searing smite

2nd level (3 slots): hold person, phantasmal force, tasha's mind whip

3rd level (3 slots): counterspell, dispel magic, enemies abound

4th level (3 slots): banishment, shadow of moil, staggering smite

5th level (2 slots): banishing smite destructive wave (necrotic)

6th level (2 slots): harm, scatter

7th level (1 slot): divine word, power word pain

8th level (1 slot): maze, sunburst

9th level (1 slot): blade of disaster

Multiattack. The Admiral makes three longsword attacks.

Longsword. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 8) slashing damage, or 10 (1d10 + 8) slashing damage if used with two hands, plus 22 (5d8) necrotic damage.

Hellfire Orb (1/Day). The Admiral hurls a magical ball of fire that explodes at a point it can see within 120 feet of it. Each creature in a 20-foot-radius sphere centered on that point must make a DC 23 Dexterity saving throw. The sphere spreads around corners. A creature takes 35 (10d6) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.

Parry. The Admiral adds 6 to its AC against one melee attack that would hit it. To do so, the death knight must see the attacker and be wielding a melee weapon.

Vengeance. The Admiral can make one melee attack against any creature in his reach that deals damage to you. You have advantage on the attack roll, and any damage dealt with this special attack ignores any damage immunity or resistance the target has.

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

Planar Leap. The Admiral casts thunder step.

Crimson Consecration. The Admiral casts life transference on the Lich he serves.

Raging Retort (Costs 2 Actions). The Admiral regains his reaction.

If the Admiral's mythic trait is active, he can use the options below as legendary actions.

Cutting Servitude. The Admiral makes one Longsword attack.

Desecrated Arcana (Costs 2 Actions). The Admiral casts a spell of 6th level or lower.

Spiteful Surge (Costs 3 Actions). The Admiral points at one creature within 60 ft. of him. The target must make a Constitution saving throw or take 5d10 force damage and  or half as much on a success.