Crab-Eel

Underwater Breathing. The eel can breathe underwater.

Electrifying Touch. Any melee attack made against this creature requires the attacker to make a DC 12 constitution saving throw or take 1d6 lightning damage.

Spellcasting. The eel is a 17-level spellcaster. Its spellcasting ability is wisdom (spell save DC 16, +7 to hit with spell attacks). The eel has following wizard spells prepared:
Cantrips (at will): Lightning Lure, Shocking Grasp, Eldritch Blast, Primal Savagery
1st level (5 slots): Witch Bolt, Chaos Bolt, Absorb Elements

 2nd level (4 slots): Dragon's Breath

3rd level (3 slots): Lightning Bolt

4th level (3 slots): Storm Sphere

6th level (2 slots): Chain Lightning

9th level (1 slot): Storm of Vengeance

Multiattack. The eel makes 2 attacks one with its claws and another with its tail.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 12 (3d6 + 2) piercing damage. 

Claws. Melee Weapon Attack: +5 to hit, range 10 ft., one target. Hit: 9 (1d8 + 4) piercing damage.

Tail. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) lightning damage and the target must make a constitution saving throw (DC 13) or be Stunned for one round.

Opportunity Attack. The eel makes one attack (claw or tail attack only) against a creature leaving its melee range.