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

Marshal the Unholy. As long as the death knight is not incapacitated, it and all undead and fiends of its choice within 30 feet of it gain advantage on saving throws against being turned or frightened.

Spellcasting. The death knight is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). 

Multiattack. The death knight makes two attacks with its longsword or one attack with its longsword and casts dispel magic.

Longsword. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) slashing damage, or 10 (1d10 + 5) slashing damage if used with two hands, plus 9 (2d8) necrotic damage.

Dispel Magic (3rd-level) (Recharge 4-6). 

Horrify (Recharge 5-6). Each creature in a 30 foot cone originating from the death knight must succeed a DC 16 Wisdom saving throw or drop whatever it is holding and become frightened for 1 minute. If a creature fails the initial saving throw by 5 or more, it is paralyzed while frightened in this way. An affected creature can repeat this saving throw at the end of each turn ending this effect on a success.

Dark Passage. The death knight gains incorporeal movement and resistance to all damage except radiant until the end of its turn. It then flies 30 feet.

Profane Smite. The next time the death knight hits a creature with a weapon attack before the end of its turn, it deals an additional 13 (3d8) necrotic damage and all creatures that have taken necrotic damage since the end of the death knight's last turn must succeed a DC 16 Constitution saving throw or be pushed 15 feet away from it and knocked prone.

Command (2nd-level). 

Staggering Parry. The death knight adds 5 to its AC against one melee attack targeting it, if this causes the attack to miss, the attacker takes 10 (3d6) psychic damage and must succeed a DC 16 Wisdom saving throw or it has disadvantage on attack rolls and ability checks, and can't take reactions until the end of its next turn.

VILLAIN ACTIONS: 

1. Aura of Dread. All creatures of the death knight's choice make saving throws with disadvantage and only regain half as many hit points from magical healing while within 30 feet of the death knight until it loses concentration (as if concentrating on a spell).

2. Hellfire Lance. All targets within a 15-foot by 150-foot line originating from the death knight must make a DC 16 Dexterity saving throw. A creature takes 21 (6d6) fire damage and 21 (6d6) necrotic damage on a failed save, or half as much on a successful one. Whether a target succeeds or fails, it cannot regain hit points until the end of its next turn.

3. Blade of Annihilation. All targets within a 25-foot cone originating from the death knight must make a DC 16 Dexterity saving throw. A target that succeeds this save takes 9 (1d8 + 5) slashing damage, plus 13 (3d8) necrotic damage. On a failure, the target drops to 0 hit points. Any creature reduced to 0 hit points by this ability immediately fails one death save and makes death saves with disadvantage until it finishes a short or long rest.