Vanth

Mimicry.  Much like the kenku the vanth can mimic any sounds it has heard, including voices. A creature that hears the sounds can tell they are imitations with a successful DC 14 Wisdom (Insight) check.

Fear Aura. Any creature within 30 feet of the vanth most make a DC 17 Wisdom saving throw or be 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 the dragon's Frightful Presence for the next 24 hours.

Spell like ability. The Vanth is a 10th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +13 to hit with spell attacks). The Vanth has the following spells prepared:
At will: Inviability, Eye Bite, Scrying.

1/Day: Teleport (self only) (does not need to roll on the spell table and must be on the same plane of existence).

3/Day: Bestow Curse, Searing Smite, Black Tentacles.

Legendary Resistance (3/Day). If the Vanth fails a saving throw, it can choose to succeed instead.

Multiattack. The vanth either makes one scythe attack or two claw attacks.

Scythe. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 17 (4d4 + 7) Slashing damage. plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vanth regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way, buried or not rises in 1d4 hours as a new Vanth.

Claw. Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 8 (1d6 + 4) Slashing damage.

The vanth 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. The vanth regains spent legendary actions at the start of its turn.

Far Step. The vanth teleports up to 60 feet to an unoccupied space it can see.

Assassinate. (Costs 2 Actions). The vanth makes one scythe attack.

Wing Attack. (Costs 2 Actions). The vanth beats its wings. Each creature within 10 feet of the vanth must succeed on a DC 17 Dexterity saving throw or take 10 (2d6 + 3) bludgeoning damage and be knocked prone. The vanth can then fly up to half its flying speed.