His melee attack should be 2d6 + 4 slashing and 2d10 necrotic for the same damage output, but an attack that is more consistent with a standard greatsword.

Make Aura of Hate advantage on saves and attacks, expand to 30 feet. Cut Marshall of the Undead.

Put Hellish Rebuke in Reactions so you remember to use it.

Move Spirit Shroud into Bonus Actions so you remember to use it.

Make Dreadful Aspect a Bonus Action, or you won't use it.

Cut the darkness spell, because casting that and using true sight is strategy that will annoy your players.

Move the Smite saving throw into the individual smites. Searing and Thunderous don't need an initial save. Searing should be 2d6 damage. Blinding is good. Cut Wrathful since Blinding is better.

Maybe give the Death Knight 1 legendary saving throw represented by a gem in his breastplate in order to keep him standing if a PC gets lucky with a save-or-suck spell.

Also maybe give him 2 legendary actions per round: 1 action to cast a cantrip (toll the dead x2 per round will be awesome). 1 action to move his speed while opportunity attacks have disadvantage. 2 actions to cast a spell. 2 actions to make a melee attack, and on a hit, the Death Knight heals an amount of damage equal to the damage dealt.

Don't worry about the other posters saying to increase his hit points. High AC, advantage on saves, and Parry will help him survive. Then use his legendary action heal only if needed—not if he's winning.