Arthas

Commander of Bones. As a bonus action, the knight can target one skeleton or zomibe 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. 

Misty Escape. When it drops to 0 hit points outside its resting place, the vampire transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that it isn't in sunlight or running water. If it can't transform, it is destroyed.
While it has 0 hit points in mist form, it can't revert to its vampire form, and it must reach its resting place within 2 hours or be destroyed. Once in its resting place, it reverts to its vampire form. It is then paralyzed until it regains at least 1 hit point. After spending 1 hour in its resting place with 0 hit points, it regains 1 hit point.

Regeneration. Arthas regains 10 hit points at the start of its turn if it has at least 1 hit point and isn't in sun light or running water. lf the vampire takes radiant damage or damage from holy water, this trait doesn't function at the start of the vampire's next turn.

Bonecraft Armor. Arthas' armor is crafted of leather and bone, held together with enmity and magic, but weakens from heavy blows. If a melee attack roll from a weapon that deals bludgeoning damage and is wielded two-handed scores a 15 or higher, have the attacker roll weapon damage even if the attack does not hit. If the damage roll is higher than 8, the armor absorbs the damage and Arthas' AC is reduced by 1 to a minimum of 15, at which point the bonecraft armor breaks. If the attack hit the armor but not Arthas, he takes no additional damage from it. If the attack hit Arthas, then he is dealt the excess damage with his normal damage resistances applying. Bludgeoning damage from non-magical weapons that aren't silvered still cause the bonecraft armor to break.

Master of the Palid Banner. While within 60 feet of the knight, any undead ally of the knight has advantage on saving throws against any effect that turns undead.

 Vampire Weaknesses. The vampire has the following flaws:
Forbiddance. The vampire can't enter a residence without an invitation from one of the occupants.
Harmed by Running Water. The vampire takes 20 acid damage if it ends its turn in running water or 10 acid damage if it's exposed to water
Stake to the Heart. If a piercing weapon made of wood is driven into the vampire's heart while the vampire is incapacitated in its resting place, the vampire is paralyzed until the stake is removed.
Sunlight Hypersensitivity. The vampire takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.

Spellcasting. The knight is an 8th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 14, +6 to hit with spell attacks). It has the following paladin spells prepared:
1st level (4 slots): command, compelled duel, hellish rebuke, wrathful smite 

2nd level (3 slots): branding smite, crown of madness, darkness, find steed, magic weapon

Multiattack. The 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.