Ananke, the Sea Witch

Amphibious. Ananke can breath air and water. 

Beguiling Presence. At the start of their turn, creatures within 120 feet of Ananke must succeed on a DC 20 Charisma saving throw or suffer psychic pain when they attempt to harm her. Whenever an affected creature makes an attack against Ananke, they take psychic damage equal to 5 - their charisma modifier (minimum 1). If a creature's saving throw is successful, the creature is immune to Ananke's Beguiling Presence for the next 24 hours.

Legendary Resistance (3.day). When Ananke fails a saving throw, she can choose to succeed instead.

Split Enchantment. When Ananke casts an enchantment spell of 1st level or higher that targets only one creature, she can have it target a second creature.

Spellcasting. Anankeis a 15th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). Ananke has following Wizard spells prepared:
Cantrips (at will): friends,message, minor illusion
1st level (4 slots): bane, command, sleep
2nd level (3 slots): hold person, mirror image, suggestion
3rd level (3 slots): fear, major image
4th level (3 slots): confusion, greater invisibility
5th level (2 slots): dominate person, geas, modify memory
6th level (1 slot):mass suggestion, programmed illusion
8th level (1 slot): dominate monster, feeblemind

Multiattack. Ananke uses her Ink Cloud, then makes 4 grasp attacks.

Grasp. Melee Weapon Attack: +9 to hit, reach 20 ft., 1 target. Hit: The target is grappled (escape DC 17). While grappled the target is also restrained.

Ink Cloud. Ananke releases ink around her, making the area within 20 feet heavily obscured.

Irresistible Offer. Whenever a creature within 60 feet of Ananke casts a spell with a verbal component, she can attempt to steal their voice. That creature must succeed on a DC 18 Charisma saving throw or they become mute, lose the ability to perform the verbal components of spells, and the spell fails.
When a creature's voice is stolen, it receives something in return. That creature rolls a d4.

1. That creature loses one speed of it's choice among walking, swimming, or flying speed, and gains a speed of equivalent amount of another of their choice. It's body may change to accommodate this.
2. That creature loses proficiency in one saving throw of their choice in which they are proficient and gain proficiency in another saving throw of their choice.
3. That player's race, sex, and alignment change to another of their choice.
4. That player may choose from 1-3.

These effects last until Ananke is killed, she chooses to release the voice, she uses this ability against another creature, or it is removed through a spell such as remove curse. If the effect lasts a year and a day, it become permanent and cannot be removed except by the spell wish.

Enter the description for how legendary actions work for your monster here.

Grasp . Ananke makes one grasp attack.

Incant. Ananke casts a single spell of 2nd level or lower with a casting time of 1 action or 1 bonus action.

Psychic Drain (costs 2 actions). Creatures charmed by, grappled by, or wearing the enchanted collars of Ananke within 60 feet that can hear her must succeed on a DC 18 Intelligence saving throw or have their maximum hit points reduced by 3. Ananke recovers 3 hit points for each creature that failed the saving throw.