Magic Resistance- The BBEG has advantage on saving throw against spells

Legendary Resistance- If BBEG would fail a saving throw, he can choose to succeed instead.

Regeneration- The BBEG regenerates 20 Hp at the beginning of his turn.

Spider Climb. The BBEG can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Trample- If the BBEG runs in a 20 foot straight line towards a character, they must make a strength save or take 5 bludgeoning damage and be knocked prone.

 Annihilating Aura. Any creature that starts its turn within 30 feet of Aceran must succeed on a DC 21 Constitution saving throw or take 15 necrotic damage.

Spellcasting. The BBEG is a 9th-level spellcaster. Its spellcasting ability is Wisdom DC 25, the BBEG gets a +9 to spell attacks. The BBEG has following Warlock, Sorcerer, Cleric and Wizard spells prepared:
Cantrips (at will): Chill Touch, Firebolt, Ray Of Frost, Eldritch Blast

1st level (at will): Wrathful Smite, Which Bolt, Guiding Bolt, Chaos Bolt

2nd level (at will): Darkness, Flaming Sphere, Heat Metal, Hold Person, Spiritual Weapon

3rd level (at will): Fireball, Hunger Of Hadar, Hypnotic Pattern, Lightning Bolt, Pulse Wave, Thunderstep

4th level (at will): Banishment, Deathward, Dimension Door, Summon Greater Demon

5th level (at will): Bigbys Hand, Destructive Wave, Immolation, Steel Wing Strike, Synaptic Static

6th level (at will): Blade Barrier, Chain Lightning, Disintegrate, Heal, Harm, Mental Prison

7th level (at will): Finger Of Death, Fire Storm, Crown Of Stars, Prismatic Spray

8th level (at will): Abi-Dalzim’s Horrid Wilting, Dark Star, Feeblemind, Incendiary Cloud, Maddening Darkness, Maze, Reality Break

9th level (at will): Blade Of Disaster, Meteor Swarm, Power Word Kill, Prismatic Wall, Physic Scream, Ravenous Void, Time Ravage, Time Stop, True Polymorph, Wish

Multi-Attack. Aceran can use its Frightening Presence then makes 5 attacks: One with its bite, three claw attacks, and 1 with its tail or Nepenthe.

Frightful Presence. Each creature of Aceran's choice that is within 120 feet of Aceran and aware of it must succeed on a DC 24 Wisdom saving throw or become frightened  for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Aceran's Frightful Presence for the next 24 hours.

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 25 Piercing damage and 10 Lightning damage. 

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 20 Slashing damage and 15 fire damage.

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 30 Bludgeoning damage.

Nepenthe. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 20 Slashing Damage and 15 Radiant Damage.

Elemental Breath (Recharge 5–6). Aceran exhales either Fire, Cold, Lightning, Acid, or Force in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 90 of that damage on a failed save, or half as much damage on a successful one.

Gorging Rush.  Immediately after using the Dash action, Aceran can make one melee attack with his horns.

Teleport. Aceran magically teleports, along with any equipment he is wearing and carrying, up to 120 feet to an unoccupied space he can see.

Throw- The BBEG can choose to grapple and throw a large or smaller enemy.

Spreading Wings. When attacked the BBEG can spread his wings and cause everyone to make a dexterity save or be knocked prone.

Teleport. Aceran magically teleports, along with any equipment he is wearing and carrying, up to 120 feet to an unoccupied space he can see.

Aceran can take 3 legendary actions, choosing from the options below.  Aceran regains spent legendary actions at the start of its turn.

Wing Attack (Costs 2 Actions) Aceran beats his wings. Each creature within 10 feet of Aceran must succeed on a DC 20 Dexterity saving throw or take 13 bludgeoning damage and be knocked prone. Aceran can then fly up to half its flying speed. 

Tail Attack. Aceran makes a tail attack.

Teleport. Aceran uses her Teleport action.

If Aceran drops to 0 hit points, he will turn to ash and pop out of a flaming tornado with 400 health.

Hells Gaze (Costs 3 Actions). Aceran looks at a creature in range and send them through hell, as if affected by the Hurl Through Hell feature.

Into The Shadows (Costs 2 Actions). Aceran looks at 3 creatures and covers them in shadowy flames dealing 30 Necrotic damage and 30 Fire damage.

The Liches Eye (Costs 1 Action). Aceran lifts his hand and creates a magical symbol on the ground within 30 feet of Aceran, trough the portal summons a lich, the lich goes after Aceran.