Amphibious. Dagon can breathe air and water.

Form of Madness. Any creature that starts its turn within 120 feet of Dagon that can see him must make a DC 23 Wisdom saving throw. On a failed save, the creature is frightened and suffers the effect of the confusion spell until the start of Dagon's next turn. If a creature's saving throw is successful, the creature is immune to Dagon's Form of Madness for the next 24 hours.

Innate Spellcasting. Dagon's spellcasting ability is Charisma (spell save DC 23). Dagon can innately cast the following spells, requiring no material components:

At will: darkness, detect evil and good, dream, freedom of movement

3/day each: dispel magic, telekinesis, teleport

1/day each: control weather, gate, symbol

Legendary Resistance (3/Day). If Dagon fails a saving throw, he can choose to succeed instead.

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

Magic Weapons. Dagon's weapon attacks are magical.

Regeneration. Dagon regains 15 hit points at the start of his turn.

Siege Monster. Dagon deals double damage to objects and structures.

Multiattack. Dagon makes five attacks: two with his tentacles, two with his claws, and one with his bite.

Bite. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 29 (3d12 + 10) piercing damage. If the target is a Large or smaller creature grappled by Dagon, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Dagon, and it takes 49 (14d6) acid damage at the start of each of Dagon's turns.

If Dagon takes 50 damage or more on a single turn from a creature inside him, Dagon must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Dagon. If Dagon dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 30 feet of movement, exiting prone.

Claw. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 26 (3d10 + 10) slashing damage.

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.

Tentacle. Melee Weapon Attack: +18 to hit, reach 40 ft., one target. Hit: 23 (3d8 + 10) bludgeoning damage. If the target is a creature, it is grappled (escape DC 26) and must succeed on a DC 24 Constitution saving throw or be poisoned for 1 minute. Until this grapple ends, the target is restrained. A poisoned creature no longer grappled by Dagon can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Dagon has ten tentacles, each of which can grapple one target.

Doomsong (Recharge 6). Dagon emits a hypnotic, rumbling song that can lure nearby creatures to his location. Each creature within 300 feet of Dagon that can hear him must succeed on a DC 23 Wisdom saving throw or be charmed by Dagon for until the song ends. While charmed, the creature must use its movement on each of its turns to move within reach of Dagon. When the creature has done so, it can't willingly move away from Dagon.

If the charmed creature takes any damage, it can repeat the saving throw, ending the effect on itself on a success. Dagon must concentrate to maintain the song (as if concentrating on a spell), which lasts for up to 1 minute.

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 his turn.

Tentacle Attack or Fling. Dagon makes one tentacle attack or uses his Fling.

Doom Drone (Costs 2 Actions). Each creature within 30 feet of Dagon that can hear him must succeed on a DC 23 Wisdom saving throw or take 22 (4d10) psychic damage.

Claws of the Deep (Costs 3 Actions). Dagon makes a claw attack against each creature within 30 feet of him.