Babilonia: the sin of lust

Flyby. Lust doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Legendary Resistance (4/Day). If Lust fails a saving throw, it can choose to succeed instead.

Absolute telepathy.  When it's concious Lust is considered permanently under the effect of the spells "Detect Thought" and "tongues", and can communicate telepatically with any creature that it can see. Because of the clairoveggence given by its telepaty, attack against Lust are made with disadvantage

Shapechanger. The fiend can use its action to polymorph into a Small or Medium humanoid, or back into its true form. Without wings, the fiend loses its flying speed. Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Lust's tongue. Lust is considered permanently under the effect of the spell Glibness. This effect cannot be dispelled or circumvented in any way except for wish spell. 

Innate Spellcasting. Lust's innate spellcasting ability is Charisma (spell save DC 24) . Lust can innately cast the following spells, requiring no  components:

(at will): Charm persons, Command, scrying, telepathy, dream, suggestion, misty step.

3/day each: Thasha's mind whip, counterspell (lv7), dispell magic (lv 7), scatter, wall of force, metamorphosis

1/day each  Synaptic static, Mass Suggestion, Mental prison, Mirage arcane,disintegrate feeblemind, illusory dragon, Power word kill, psychic scream.

Alien Mind. If a creature tries to read Lust's thoughts, target him with enchantement spells or deal psychic damage to it, that creature must succeed on a DC 24 Intelligence saving throw or be stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Lust can't be targeted by divination magic, perceived through magical scrying sensors, or detected by abilities that sense fey.

Enthralling Madness. Whenever a creature fails an Intelligence or Wisdom saving throw against one of the Lust's effects, it suffers one level of exhaustion. If a creature reaches six levels of exhaustion, it doesn't die; it instead becomes a thrall under Lust's control, and all its exhaustion is removed. Only a wish spell or the destruction of the spawn can free the thrall from this control.

Parasite: When Lust dies it can chose a creature within 120 ft, that creature must succeed a DC 24 charisma saving throw, if it fails Lust take the controll of his body, can use all its abilities, as well as their own.If the creature succeed it's immune to this abilities for 1 day. If the creature succeed the saving throw Lust can chooes another creature as an action. Lust dies if it hasn't a body for 1 minute.

Multiattack: Lust uses Rend the Mind two times, then it attack two times with Fairy Touch. 

Fairy Touch. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) bludgeoning damage plus 27 (6d8) thunder damage

Rend the Mind. Lust uses one of the following psionic powers of its choice, targeting one creature it can see within 120 feet of it; it can't use the same effect two rounds in a row:

1. Crush Will. The target must succeed on a DC 24 Charisma saving throw or be overwhelmed with despair for 1 minute. During this time, it can’t attack or target any creature with harmful spells or other effects. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

2. Devour Mind. The target must make a DC 24 Intelligence saving throw, taking 49 (11d8) psychic damage on a failed save, or half as much damage on a successful one. Also on a failure, roll 3d6; if the total equals or exceeds the target's Intelligence score, that score is reduced to 0. The target is stunned until it regains at least one point of Intelligence. The reduction lasts until the target finishes a long rest.

3. Dominate. The target must succeed on a DC 24 Wisdom saving throw or be charmed by Lust for 1 minute or until the target takes damage. Lust can issue telepathic commands to the charmed creature (no action required), which it does its best to obey.

4. Inner-Reconstruction. The target must make a DC 24 Constitution saving throw, taking 66 (12d10) necrotic damage on a failed save, or half as much damage on a successful one. A humanoid reduced to 0 hit points by this damage dies and transforms at the start of Lust's next turn. If the humanoid had 4 or fewer Hit Dice, it becomes a pixie. If it had 5 or more Hit Dice, it becomes a Succubus or Incubus. The humanoid is under Lust’s control and acts immediately after the spawn in the initiative order. The target can’t be returned to its original form by any means short of a wish spell.

Charm. One humanoid Lust can see within 30 feet of it must succeed on a  Wisdom saving throw or be magically charmed for 1 day. The charmed target obeys Lust verbal or telepathic commands. If the target suffers any harm or receives a suicidal command, it can repeat the saving throw, ending the effect on a success. If the target successfully saves against the effect, or if the effect on it ends, the target is immune to this Charm for the next 24 hours.

Etherealness. Lust magically enters the Ethereal Plane from the Material Plane, or vice versa.

Lust has 3 legendary action, and it can use a legendary action at the end of an enemy's turn, it regain all the legendary action at the beginning of its turn. 

Cast (Cost 2 actions): Lust cast a spell

Teleport.  Lust magically teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see.

Charming Aura(Costs 2 Actions). Lust emits his power in order to charm his enemy. Each creature of its choice in a 10-foot radius must make a DC 24 Winsdom saving throw, taking 14 (4d6) psychic damage and becoming charmed on a failed save, or half as much damage and not becoming charmed on a successful one. When a creature is charmed in this way it moves in the way Lust want.