Amphibious. The monarch can breathe air and water.

Eldritch Restoration. If destroyed, the monarch gains a new body in 3d10 days, reviving with all its Hit Points in the Far Realm or another location chosen by the DM.

Endless Malevolence (Recharges after a Short or Long Rest). If the monarch would be reduced to 0 hit points, its current hit point total instead resets to 182 hit points, it recharges its Enslave action, and it regains any expended uses of Legendary Resistance. Additionally, the monarch can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 13,000 XP (26,000 XP total) for defeating the monarch after its Endless Malevolence trait activates.

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

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

Mucus Cloud. While underwater, the monarch has Advantage on saving throws made to avoid or end the Grappled condition. Additionally, it is surrounded by mucus. Constitution Saving Throw: DC 16, each creature in a 5-foot Emanation originating from the monarch at the end of the monarch's turn. Failure: The target is cursed. Until the curse ends, the target’s skin becomes slimy, the target can breathe air and water, and it can’t regain Hit Points unless it is underwater.

While the cursed creature is outside a body of water, the creature takes 6 (1d12) Acid damage at the end of every 10 minutes unless moisture is applied to its skin before those minutes have passed.

Probing Telepathy. If a creature the monarch can see communicates telepathically with the monarch, the monarch learns the creature’s greatest desires.

Multiattack. The monarch makes three Tentacle attacks and uses either Consume Memories, Dominate Mind, or Spellcasting to cast Tasha's Mind Whip if available.

Tentacle. Melee Attack Roll: +11, reach 20 ft. Hit: 15 (2d8 + 6) Bludgeoning damage. If the target is a Huge or smaller creature, it has the Grappled condition (escape DC 16) from one of eight tentacles.

Consume Memories. Intelligence Saving Throw: DC 18, one creature within 60 feet that is Charmed or Grappled by the monarch. Failure: 14 (4d6) Psychic damage. Success: Half damage. Failure or Success: The monarch gains the target’s memories if the target is a Humanoid and is reduced to 0 Hit Points by this action.

Domination (4/Day). Wisdom Saving Throw: DC 18, one creature the monarch can see within 60 feet. Failure: The target has the Charmed condition until the monarch dies or is on a different plane of existence from the target. While Charmed, the target acts as an ally to the monarch and is under its control while within 90 feet of it. In addition, the monarch and the target can communicate telepathically with each other over any distance.

Spellcasting. The monarch casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

3/Day Each: Command (level 3 version), Compulsion, Enemies Abound, Enthrall, Tasha's Mind Whip
1/Day Each: Antipathy/Sympathy, Befuddlement, Hold Monster (level 6 version), Mass Suggestion, Psychic Scream, Synaptic Static

Command Thrall. The monarch commands one creature Charmed by it to use its Reaction to make an attack against another creature of the monarch's choice.

Slave Shield. Trigger: A creature the monarch can see makes an attack roll against it. Response: The monarch chooses an ally within 60 feet of itself, and the ally becomes the target of the attack instead.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the monarch can expend a use to take one of the following actions. The monarch regains all expended uses at the start of each of its turns.

Corrupted Water. Wisdom Saving Throw: DC 16, each enemy the monarch can see underwater within 90 feet. Failure: 14 (4d6) Psychic damage. Success: Half damage. Failure or Success: The monarch can’t use this action again until the start of its next turn.

Lash. The monarch makes one Tentacle attack.

Psychic Drain. If the monarch has at least one creature Charmed or Grappled, it uses Consume Memories and regains 11 (2d10) Hit Points.

If the monarch's Endless Malevolence has activated in the last hour, it can use the options below as legendary actions.

Assert Control. The monarch uses spellcasting to cast Command (level 3 version) if available. The monarch can't use this action again until the start of its next turn.

Flee. The monarch moves up to its speed without provoking Opportunity Attacks.

Surging Tide. Strength Saving Throw: DC 16, each enemy within 20 feet of water the monarch can see within 90 feet. Failure: The target is pulled into the water, and has the Prone condition. Failure or Success: The monarch can’t use this action again until the start of its next turn.