Vadataj

Aura of Drowning. A creature within 10 feet of the vadataj can’t breathe and must hold its breath or begin to suffocate. In addition, a creature that starts its turn in the aura and doesn’t have a swim speed or isn’t undead has its movement speed halved until the start of its next turn.

Magic Resistance. The vadataj has Advantage on saving throws against spells and other magical effects.

Turn Immunity. The vadataj is immune to effects that turn undead.

Unusual Nature. The vadataj doesn’t require air, food, drink, or sleep.

Multiattack. The vadataj uses Drowning Visions if it can, Phantasmal Terror, or Spellcasting. It then makes two attacks with its Cursed Greatsword.

Cursed Greatsword. Melee Weapon Attack Roll: +11, reach 5 ft. Hit: 12 (2d6 + 5) Slashing damage plus 22 (4d10) Necrotic damage. If the target is a creature, it must succeed on a DC 18 Wisdom saving throw or be cursed with vadataj drowning. The cursed target gains the Aura of Drowning trait. The curse lasts until removed by a remove curse spell or other magic.

Drowning Visions (Recharge 4–6). The vadataj magically conjures forth horrible sensation of fatally drowning in the minds of creatures in a 60-foot cone. Each creature in that area must succeed on a DC 18 Wisdom saving throw or be frightened of the vadataj until the end of its next turn. While a creature is frightened in this way, its speed is halved, and it is incapacitated.

Phantasmal Terror. The vadataj targets one creature it can see within 120 feet. The target must succeed on a DC 18 Wisdom saving throw, or it takes 16 (3d10) Psychic damage and is frightened of the vadataj until the end of its next turn.

Final Gurgle (1/Day). The vadataj inhales deeply, drawing the breath away from creatures around it, before finally releasing the horrible gurgling sound of hundreds of drowning creatures. This gurgle has no effect on constructs, oozes, and undead. All other creatures within 30 feet of the vadataj that can hear it must make a DC 18 Constitution saving throw. On a failure, a creature drops to 0 hit points. On a success, a creature becomes immune to the Final Gurgle of all vadataj for 24 hours.

Spellcasting. The vadataj casts one of the following spells, requiring no Material components and using Intelligence as the spellcasting ability (spell save DC 18, +11 to hit with spell attacks):

At Will: Blur, Disguise Self, Hallucinatory Terrain (as an action, changing the terrain to look underwater), Invisibility, Phantom Steed (as an action)
3/Day Each: Mislead, Programmed Illusion, Seeming

1/Day Each: Mirage Arcane (as an action, changing the terrain to look underwater)