Stench of Undeath. Any creature that starts it’s turn within 10 feet of the zombie knight must succeed on a DC15 constitution saving throw or be poisoned until the start of its next turn. On a successful save, the creature is immune to the zombie knight’s Stench of Undeath for 24 hours.

Spellcasting. The Zombie Knight is a 12-level spellcaster. Its spellcasting ability is Wisdom (Spell Save DC 15, +7 to hit with spell attacks). The Zombie Knight has the following spells prepared: 
 Cantrips (at will): Booming Blade, Poison Spray, Chill Touch 

1st level (3 slots): Cause Fear, Ray of Sickness

2nd level (2 slots): Ray of Enfeeblement

3rd level (2 slots): Animate Dead 

Undead Fortitude. If damage reduces the Zombie Knight to 0 hit points, it must make a constitution saving throw with a DC of 5+ the damage taken, unless the damage is radiant or from a critical hit. On a success the Zombie Knight drops to 1 hit point instead. 

Zombie Mastery. The Zombie Knight can exert control over lesser zombies. When a zombie not already under the control of the Zombie Knight starts its turn within 10 feet of the Zombie Knight, it must succeed on a DC15 wisdom saving throw or be subjected to the mental commands of the Zombie Knight, as if summoned by Animate Dead.

Advanced Multi-attack. The Zombie Knight makes 3 attacks when they take the attack action. It can substitute a cantrip for one of its attacks.

Longsword. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 14 (2d8 + 5) Slashing Damage plus 7 (2d6) Necrotic Damage. 

The Zombie Knight has 3 Legendary actions per turn, 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 Zombie Knight regains all spent legendary actions at the start of its turn.

Command Undead (Costs 1 Action). Command a zombie under your control to move up to half its speed and make a single attack.

Death Strike (Costs 1 Action). Make one weapon or spell attack against a creature in range. 

Aura of Death (Costs 2 Actions). Each living creature within visual range of the Zombie Knight must make a Constitution saving throw (DC15). On a failed save, the creature’s hit point maximum is reduced by 1d6. If a creature’s hit point maximum is reduced to 0 by this effect, the creature dies, and is immediately brought back as a zombie. This reduction lasts until the creature takes a long rest.

Legendary Resistance (1/day). When the Zombie Knight makes a saving throw and fails, it can instead choose to succeed the saving throw.