Undead Nature. An Abyssal Knight doesn't require air, food, drink, or sleep.
Magic Resistance. The Abyssal Knight has advantage on saving throws against spells and other magical effects.
Commander of Bones. As a bonus action, the knight can target one skeleton or zombie it can see within 30 feet of it. The target must make a DC 14 Wisdom saving throw. On a failed save, the target must obey the knight’s commands until the knight dies or until the knight releases it as a bonus action. The knight can command up to twelve undead at a time this way.
Undead Thralls. When the Abyssal Knight casts animate dead, it can target one additional corpse or pile of bones, creating another zombie. Undead created by the Abyssal Knight become Greater Zombies.
Spellcasting. The Abyssal Knight is an 18th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). The Abyssal Knight has the following wizard spells prepared:
Cantrips (at will): mage hand, prestidigitation, ray of frost
1st level (4 slots): detect magic, magic missile, shield, thunderwave
2nd level (3 slots): acid arrow, detect thoughts, invisibility, mirror image
3rd level (3 slots): animate dead, counterspell, dispel magic, fireball
4th level (3 slots): blight, dimension door
5th level (3 slots): cloudkill, scrying
6th level (1 slot): disintegrate, globe of invulnerability
7th level (1 slot): finger of death, plane shift
8th level (1 slot): dominate monster, power word stun
9th level (1 slot): power word kill
Multiattack. The Abyssal knight attacks twice with one of its weapons.
Greatsword. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.
Longbow. Ranged Weapon Attack: +4 to hit, range 150/600 ft., one target. Hit: 5 (1d8 + 1) piercing damage.
Leadership (Recharges after a Short or Long Rest). For 1 minute, the knight can utter a special command or warning whenever a nonhostile creature that it can see within 30 feet of it makes an attack roll or a saving throw. The creature can add a d4 to its roll provided it can hear and understand the knight. A creature can benefit from only one Leadership die at a time. This effect ends if the knight is incapacitated.