Archdevil Lilith

Fear Aura. Any creature hostile to Lilith that starts its turn within 20 feet of her must make a DC 21 Wisdom saving throw, unless Lilith is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to her Fear Aura for the next 24 hours.

Eldrich Invocations: Agonizing Blast,  Aspect of the moon, Beast Speech, Beguiling Influence, Bewitching Whispers, Chains of Careri, Dreadful Word, Eldrich Mind, Eldrich Spear, Gaze of Two Minds, Ghostly Gaze, Gift of the Depths, Grasp of Hadar, Lance of Lethargy, Maddening Hex, Mask of Myriad Forms, Relentless Hex, Repelling Blast, Whispers of the grave, Witch Sight, Visions of Distant Realms.

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

Magic Weapons. All Lilith's attacks are considered magical for purposes of resistance.

Innate Spellcasting. Lilith can innately cast all her known spells, requiring no material or verbal components.

Flight. Lilith can move up to 120m in any direction using flight.

Infernal Hierarchy.  At the start of battle Tieflings, fiends, creatures with Infernal blood or warlocks in pacts with infernal beings must make a DC 21 Charisma saving throw or be Charmed for 5 turns.

Planewalker. During movement Lilith can shift between planes. Lilith will be immune to all damage and effects but will be unable to attack or otherwise harm other creatures. Conditions inflicted by Lilith on other creatures will end immediately when this ability is used.

Teleportation. Lilith can user her movement to teleport to a unoccupied space in any direction. Teleportation  counts as movement and does not provoke reactions or opportunity attacks.

Fiendish Matron. Allied Tieflings, fiends, creatures with Infernal blood, or warlocks in pacts with infernal beings gain advantage on attack rolls.

Celestial Durability. Lilith recovers 5d20 HP at the start of every turn with a minimum of 20 HP.

Multi-attack. Lilith makes three attacks:  Two with her claws, and one with her tail.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 17 (4d20 + 14) slashing damage. The target must succeed on a DC 21 Constitution saving throw or become poisoned. While poisoned in this way, the target can't regain hit points, and it takes (6d12) points of poison damage per turn. for 5 turns poison damage at the start of each of its turns. The poisoned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage.

Lilith is a 20th-level spellcaster. Her spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). Lilith knows the following spells.

Cantrips (at will): Fire Bolt, Mage Hand, Presdigitation, Chill Touch, Eldrich Blast, Infestation, Mage Hand, Friends, Minor Illusion, Toll the Dead, Resurrection, Guidance

1st Level (At will): Charm Person, False Life,  Arms of Hadar, Silvery Barbs, Hellish Rebuke, Hex, Unseen Servant, Comprehend Language, Inflict Wounds

2nd Level (At will):  Misty Step, Invisibility, Mind Spike, Darkness, Enthrall, Invisibility, Mirror Image, Ray of Enfeeblement, Spider Climb, Suggestion, Lesser Restoration

3rd Level (At will): Fireball, Animate Dead, Counterspell Dispel Magic, Haste, Antagonize, Fear, Fly, Gaseous Form, Hunger of Hadar, Intellect Fortress, Remove Curse, Summon Undead, Vampiric Touch, Remove Curse, 

4th Level (7 slots): Blight, Charm Monster, Dimension Door, Elemental Bane, Summon Abberration, Hallucinatory Terrain 

5th Level (7 slots): Dominate Person Insect, Plague, Immolation, Cloudkill, Contact Other Plane, Danse Macabre, Hold Monster, Infernal Calling, Enervation, Contagion, Greater Restoration

6th Level (5 slots): Disintegrate, Circle of Death Investiture of Flame, Scyring, Arcane Gate, Eyebite, Create Undead, Flesh to Stone, Mass Suggestion, Soul cage, Summon Fiend, Harm

7th Level (5 slots): Finger of Death,  Delayed Blast Fireball, Etherealness, Dream of the Blue Veil, Plane Shift, Power Word: Pain

8th Level (4 slot): Dominate Monster, Abi-Dalzim’s Horrid Wilting , Feeble Mind, Demiplane, Maddening Darkness, Power Word: Stun

9th Level (3 slot): Power Word Kill, Wish, Time Stop, Astral Projection, Foresight, Inprisonment, True Polymorph, True Resurrection

Infernal Gift. Target recovers HP equal to 2d20 with and is granted temporary health points equal to the HP recovered. If the target's HP hits 0 they can roll a DC 17 constitution save to recover 2d20 points of HP, this effect can be used once per long rest.
Once per Long the target may cast the spell Finger of Death at its highest level without the use of any material or verbal components. If the target of this spell dies, it will gain the effect of Mistress of Souls and be enthralled to it's killer.
The target of this spell is bound to Lilith body and soul and must obey her every command or suffer immense pain and/or death. Target will take 10d20 necrotic damage for disobedience. This damage cannot be reduced or avoided in anyway and ignores resistance and immunities.
Infernal Gift effects lasts for 24 hours.

Flame Matron's Boon. Grants all allied creatures within 20m the effects of the spell Haste. Spellcasters may cast one spell at its highest level without the use of a spell slot, or any components, this free cast can only be used once per short rest. Creatures afflicted by this spell will also receive a +5 bonus to attack and damage rolls. Effect ends after long rest.
This spell may only be used safely every 24 hours. It can be used before then but  all targets must roll a DC 20 Constitution save. On a successful save they gain the effect of Flame Matron's Boon. Successful save roll increases +5 for every successful save. On a failed save they take 3d10 necrotic damage, cannot be given Flame Matron's boon for another 24 hours. 

Mistress of souls. When a target dies, Lilith can claim them body and soul. The target becomes a zombie with half of its maximum HP. Target will retain all of its gear, spells and spell slots, and abilities while it is a zombie. Upon death, the zombie disintegrates into ash. Target of this spell can only be revived with True Resurrection and/or Wish.

The End. Upon defeat Lilith unleashes the last of her power all targets within a 20m radius make a DC 25 Intelligence saving throw. On a failed save the targets will take 6d20 fire damage + 6d20 necrotic damage. On a successful saving throw, the target takes half damage.

Lilith starts with 6 legendary action slots. She can only use one legendary action per turn. She recovers one legendary action slot per turn.

Whispers of the grave. All hostile creates within 60 feet (18m) of take 4d12 necrotic damage and will be not able to recover hit points at all until Lilith's next turn. (Costs 1 Legendary Action)

Sulfur and Blood. Create a patch of heated earth within a target area with 20m. The ground becomes incredibly hot and the stench of smoke and sulfur poison the air under it. Targets within the radius of the attack must make a DC17 Constitution save, on a failed save the target takes 5d10 fire damage, 5d10 necrotic damage, and 5d10 poison damage. Target is, poisoned and movement speed is reduced to 0. On a successful saving throw damage is reduced by half and no other effects occur. The patch of sulfuric earth counts as difficult terrain.

Inferno. All creatures within 10m must roll a DC 17 Constitution saving throw. On a failed save the target takes 4d15 fire damage. At the start of each turn the target must roll the DC 17 constitution saving throw or take 1d15 fire damage from residual flames. On a successful save the target will only take half damage and no other effects are suffered. (Costs 2 legendary actions)

True form. Lilith unleashes her true form. In this form she is immune to all non-magical attacks, and resistant to all damage.
Lilith recovers HP equal to 10d10 with a minimum of 40 HP recovered and gains temporary HP equal to the same as the HP healed. All Lilith's melee attacks have advantage.
Lilith becomes Gargantuan size. This lasts 10 turns (Costs 3 Legendary actions)

Lilith starts with 6 Mythic action slots. She can only use one legendary action per turn. She recovers one Mythic action slot per turn.

Queen of Phlegethos: All area within a 120ft radius of Lilith becomes heavily obscured as a thick, black, choking smog fills the air. The smog is flammable, so all fire based attacks have advantage on attack and damage rolls.
A plume of molten lava shoots out of a space 20m wide. All targets in the area take 5d10 fire damage. The lava rains back down to earth causing 2d10 fire damage to all targets within a 20m radius of  the plume for 5 turns.

Hellfire. Target must make a DC 17 Constitution saving throw on a failed save take 6d20 fire damage. Half damage is taken on a successful save. (Costs 2 Mythic Actions)

Wild Hellfire. Grants target of Lilith's choice one use of the Hellfire mythic action on any target other than herself. (Costs 1 Mythic action)

Apocalypse.  The ground quakes and spews forth lava, the souls of the damned reach for their prey as the skies batter the Earth.  The area inside the 45m radius of Lilith becomes difficult terrain as the ground shakes violently, all structures take 7d20 force damage collapsing if it breaks. Damned souls grab at the feet of the unfortunate victims of this attack immobilizing them until their next turn. Plumes of lava spew forth from the earth dealing 6d20 fire damage. 1d20 meteors fall from the sky (minimum of 10) dealing 3d20 damage each with a minimum damage of 10 per meteor
All targets within a 45m radius of Lilith must make a DC 21 Constitution save. On a successful save the target takes half damage.

This action consumes all her Mythic action slots, the legendary action "True Form" must be activated.