Amphibious. The abomination can breathe air and water.

Legendary Resistance (3/Day). If the abomination fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its tentacles.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 3 (1d6) poison damage.

Tentacle. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage.

Frightful Presence. Each creature of the abomination's choice that is within 120 feet of the abomination and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the abomination's Frightful Presence for the next 24 hours.

Poison Breath (Recharge 5–6). The abomination exhales a poison gas in a 20-­foot cone. Each creature in that cone must make a DC 18 Constitution saving throw, taking 54 (10d8) poison damage on a failed save, or half as much damage on a successful one and another DC 12 Constitution saving throw if they remain in that area on their next turn, taking 27 (5d8) poison damage on a failed save, or no damage on a successful one.

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

Detect. The abomination makes a Wisdom (Perception) check.

Tentacle Attack. The abomination makes a tentacle attack.

Thrash (Costs 2 Actions). The abomination thrashes. Each creature within 10 feet of the dragon must succeed on a DC 17 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. This uses up the abomination's movement.