Aspect of Dagon, Prince of Depths

Amphibious. Dagon can breathe air and water. 

Freedom of Movement. Dagon ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled. 

Prince of Devils. Dagon deals double damage to blessed objects and/or those carrying them

Spellcasting. Dagon, has an innate spellcasting ability. Its spellcasting ability is Wisdom (spell save DC 22, +14 to hit). Dagon has the following spells prepared at all times:

Cantrips [at will]: Eldritch Blast, Poison Spray, Chill Touch

1st level [x3/day each]: Hellish Rebuke, Hideous Laughter, Magic Missile, Charm Person

2nd level [x2/day each]: Enthrall, Ray of Enfeeblement, Darkness, Detect Thoughts, Blindness/Deafness

3rd level [x1/day each]: Fear, Slow, Counterspell, Bestow Curse

Multiattack. Dagon can strike with both hands in one turn, or use fling

Eldritch Claw. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. Dagon can grapple with both of his hands. 

Fling. One Large or smaller object held or creature grappled by Dagon is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone. 

Dagon 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. Dagon regains spent legendary actions at the start of its turn. 
Eldritch Claw or Fling (Costs 1 Action). Dagon can use its Eldritch Claw twice or fling once
Ray of Enfeeblement (Costs 2 Actions). Dagon uses Ray of Enfeeblement. 
Prince of Depths (Costs 3 Actions). Dagon summons 3 monsters of CR 10 or lower to his aid. They can be placed on any open space within 15 feet of Dagon.