Basileus Marilus of the Roil

Legendary Resistance (3/day). If Basileus would fail a saving throw, he can choose to succeed instead.

Regeneration. Basileus regains 10 hit points at the start of his turn as long as he has 1 or more hit points. He instead regains 20 hit points at the start of his turn as long as he has one or more hit points and is fully submerged in water.

Emissary of the sea. Basileus can communicate simple ideas with beasts that can breathe water. 

Guardian of the Depths. Basileus ignores any of the drawbacks caused by underwater environments.

Quicksilver. Quicksilver is a +2 Magical Trident that deals an extra 4d6 lightning damage on a hit. Quicksilver grants Basileus the ability to ignore resistance to lightning damage, and to treat lightning damage as resistance. As a bonus action, Basileus may summon Quicksilver to his hand or teleport to Quicksilver's location if it is within 120 ft. of him. Quicksilver acts as Basileus's Spellcasting focus, and it has 12 charges on it. Basileus can expend charges to cast the following spells: Control Water (2 charges), Thunderstep (3 charges), Storm Sphere (4 charges), Freedom of Movement (4 charges), Control Winds (4 Charges), Wall of Ice (5 charges), Chain Lightning (5 charges), Tsunami (7 charges), and Control Weather (8 charges). Quicksilver regains 2d6 charges at dawn.

Magic Resistance. Basileus has advantage on saving throws against spells and other magical effects.

Evasion. When Basileus is subjected to an effect that allows him to make a Dexterity saving throw to take only half damage, he instead takes no damage if he succeeds on the saving throw, and only half damage if he fails.

Spellcasting. Basileus is an 20th-level spellcaster. Hiss spellcasting ability is Intelligence (spell save DC 19, +11 to hit with spell attacks). Basileus can cast disguise self and misty step at will and has the following wizard spells prepared:

Cantrips (at will): Ray of Frost, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): detect magic, shield, mage armor*, thunderwave

2nd level (3 slots): detect thoughts, mirror image, shatter

3rd level (3 slots): counterspell, fly, lightning bolt, wind wall

4th level (3 slots): banishment, Ice Storm, stoneskin*

5th level (3 slots): cone of cold, scrying, Conjure Elemental (Air or Water only).

6th level (2 slots): globe of invulnerability

7th level (2 slots): teleport

8th level (1 slot): control weather, mind blank*

9th level (1 slot): storm of vengeance

*Basileus casts these spells on itself before combat.

Multiattack. Basileus makes three Quicksilver attacks, then casts a cantrip if able.

Quicksilver. Melee or Ranged Weapon attack: +8 to hit, reach 5 ft. or range 20/120ft., one target. Hit: 9 (1d8 + 5) piercing damage plus an additional 14 (4d6) lightning damage. 

Basileus can take 3 legendary actions, choosing from the options below.

Cantrip. Basileus casts a cantrip.

Misty Step. Basileus casts Misty Step.

Quicksilver (Costs 2 Actions). Basileus makes one Quicksilver attack.

Roiling Vortex (Costs 3 Actions). Each creature within 60 feet of Basileus makes a DC 18 Constitution saving throw. On a failed save, a creature takes 10 (3d6) cold damage and 10 (3d6) lightning damage, is pushed 15 feet away from Basileus, and is knocked prone. On a successful save, the creature takes half as much damage and isn’t pushed or knocked prone.