Argarak, The Risen

Violent Rush. When Argarak takes the Dash action, it can make an attack as a bonus action. If that attack lands, it deals an extra 6 (1d10) damage for each 10 ft moved in a straight line Argarak moved.

Magic Resistance. Argarak has advantage against saving throws against spells and all magical effects

Legendary Resistance (6/day).  When Argarak  fails a saving throw, it can use 1 legendary resistance to succeed instead.

Turn Immunity. Turn undead effects do not work on Argarak.

Steady Body. Argarak can not be forced to be moved by creatures smaller than himself.

Aura of Fear. Non-Undead creatures with in 30 ft of Argarak have a penalty to saving throws equal his charisma bonus.

Spellcasting.  Argarak is an 18th Level Paladin. His spellcasting ability is Charisma (DC:24, +12 to hit with spell attacks). It has the following paladin spells prepared:

1st level (4 slots): Searing Smite, Shield of Faith, Wrathful Smite, 
Thunderous Smite

2nd level (3 slots): Branding Smite, Aid

3rd level (3 slots):  Blinding Smite, Spirit Shroud

4th level (3 slots): Death Ward, Staggering Smite

5th level (1 slots): Banishing Smite, Geas

Multiattack. Argarak can make up to 3 attacks with Veil-Slicer.

Veil-Slicer. Melee Weapon Attack: +15 to hit, reach 10 ft., 1 target. Hit: 19 (2d10 + 8) Slashing damage plus 18 (4d8 necrotic) damage. Necrotic Damage dealt by Viel-Slicer cannot be healed until Argarak is killed or after a full week has passed.

Hellfire Orb (1/Day). Argarak hurls a magical ball of fire that explodes at a point it can see within 120 feet of it. Each creature in a 20-foot-radius sphere centered on that point must make a DC 18 Dexterity saving throw. The sphere spreads around corners. A creature takes 35 (10d6) fire damage and 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one. 

Hot-Blooded Rush (Recharge 6): Argarak can use the Dash Action as a Bonus Action.

Argarak take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Argarak regains spent legendary actions at the start of his turn.

Detect. Argarak makes a Wisdom (Perception) check.

Bum-rush. Argarak moves up to half his movement speed, it can also make an attack.