Alastor, Demon Lord of Lust

Devilish Weapons. All Alastor's attack are magical. On a hit, the scimitar deals an additional 3d6 fire damage (included in the attack).

Magic Resistance. Alastor has advantage on saving throws against spells and other magical effects.

Charming Presence. Any creature of Alastor's choice that is within 60 feet of Alastor and can see him must make a DC 22 WIS saving throw. On a failed save, the creature is charmed by Alastor for 1 minute. While charmed, the creature has disadvantage on attacks against him. An affected creature can repeat it's saving throw at the end of each of their turns. The effect ends if the creature can no longer see Alastor. After the effect has ended or a creature succeeds on the saving throw, that creature is unaffected by this effect for the next 24 hours.

Create Portal. Alastor takes 1 minute to create a portal to the nine hells, or from the nine hells to the material plane.

Legendary Resistance (3/Day). If Alastor fails a saving throw, he can choose to succeed instead.

 Spellcasting. Alastor is a 20th level spellcaster. His spellcasting ability is Charisma (spell save DC 22, +16 to hit with spell attacks, spell modifier +8).

He can cast the following spells:

Counterspell, Fireball (9th level), Greater Invisibility, Scrying, Zone of Truth: at will

Banishing Smite (5th level): 3/long rest

Modify Memory (9th level): 1/long rest

Multiattack: three attacks with his scimitar or two firebolt attacks. 

Scimitar. Melee Weapon Attack: +15 to hit, reach 10 ft., 1 target. Hit: 15 (2d6 + 7) [slashing] damage plus 10 (3d6) fire damage.

Firebolt. Ranged Spell Attack: +18 to hit, range: 120 feet, 1 target. Hit: 26 (5d10) fire damage. 

Face of a Loved One. Alastor can force a creature he can see to make a DC 22 WIS saving throw by changing his appearance to seem as if he were the target's most loved person. On a failed save, the target is charmed by Alastor for 1 minute and while charmed, the target considers Alastor to be the love of his life. The target can repeat the saving throw at the and of each of their turns and has advantage against this effect if Alastor is their enemy. Once a creature succeeds on this save, they are immune to this effect for the next 24 hours. Only one creature can be charmed in this way at a time.

Kiss of a Loved One. As an action, Alastor can kiss a creature charmed by the Face of a Loved One or a willing creature. The target must make a DC 22 Constitution saving throw against this magic, taking 64 (10d10+10) psychic damage on a failed save or half as much on a successful one. The target's hit point maximum is reduced by an amount equal to the damage taken. This lasts until the target finishes a long rest. The target dies if this effect reduces its HP maximum to 0.

 Paralyzing Presence. Any creature of Alastor's choice that is within 60 feet of Alastor and can see him must make a DC 22 WIS saving throw. On a failed save, the creature is paralyzed by Alastor until the end of it's next turn. After the effect has ended or a creature succeeds on the saving throw, that creature is unaffected by this effect for the next 24 hours.

Counterspell.

Teleport. As a reaction to taking damage, Alastor teleports up to 60 feet in any direction.

Alastor can 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. Alastor regains spent legendary actions at the start of its turn.

Greater Invisibility. Alastor casts Greater Invisibility on himself.

Scimitar. Alastor makes a scimitar attack.

Firebolt. Alastor casts Firebolt.

Kiss of a Loved One (costs 2 actions).

Flaming Wings (costs 2 actions). Alastor beats his wings. Each creature within 15 feet of him must succeed on a DC 22 Dexterity Saving throw or take 15 (2d6 + 8) fire damage and be knocked prone. Alastor can then fly up to half his movement speed.