Corrupted Whale

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

Amphibious. The dragon can breathe air and water.

Siege Monster. The corrupted Whale deals double damage to objects and structures.

Multiattack. The Corrupted whale can make two attack per turn, one with it's slam and one with its tail swipe.

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit:  21 (3d10 + 5) bludgeoning + 11 (3d6) necrotic damage. 

Tail swipe. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 22(4d8 + 5) Bludgeoning + 7 (2d6) necrotic damage. On hit the target has to make a DC 18 strength saving throw or be knocked prone.

Corrupted Wail (recharge 5-6) The corrupted whale lets a terrifying cry. Every non-corrupted enemy within 90ft. has to make a dc 18 con saving throw. On a failure the affected creature take 4d6 necrotic damage and gets stunned until the end of the corrupted whale's next turn. On a succes, the creature only take half damage.

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

Regeneration(2 charges). The corrupted whale turns some of it's energies inward to heal 22 (4d10) hp

Ramming Speed. The corrupted whale can move 60ft. in a straight line, stopping if it hits something. Anything that gets hit has to make a dc19 strength save or take 4d10 bludgeoning damage.