Arcane Whale

Amphibious. The whale can breathe both air and water.

 Spellcasting. The whale is a 18st-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +9 to hit with spell attacks). The acolyte has following sorcerer spells prepared:

Cantrips (at will): dancing lights, light, mage hand (the hand is invisible), prestidigitation

1st level (3 slots): charm person, shield, thunderwave

2nd level (3 slots): blur, levitate, suggestion

3rd level (3 slots): fear, gaseous form, slow

4th level (2 slots): confusion, polymorph

5th level (2 slots): animate objects, creation

6th level (2 slots): disintegrate, globe of invulnerability

7th level (2 slots): etherealness, teleport

8th level (2 slots): dominate monster, power word stun

9th level (1 slot): time stop

Multiattack. The whale makes three attacks: two with its Tail Slam, one with its Splash Water.

Tail Slam. Melee Weapon Attack: +7 to hit, reach 15 ft., 5 targets. Hit: 37 (6d8 + 20) bludgeoning damage. 

Splash Water. Ranged Weapon Attack: +5 to hit, range 40/80 ft., 10 targets. Hit: 41 (8d8 + 10) cold damage.

Flip Boat. The whale attempts to flip a boat (if any in the vicinity). The whale rolls a Strength save (DC 15). Everyone in the boat and conscious rolls a Dexterity save (DC 15) to jump out of the boat. If they succeed they take 5 cold damage, if they fail they take 10 bludgeoning damage and 5 cold damage.

Shield. The whale casts the shield spell.

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

Tail Slam. The whale uses its tail slam attack.

Flip Boat. (Costs 2 Actions) The whale uses its Flip Boat action.

Spell. (Costs 3 Actions) The whale casts a spell of 7th level or lower.