Echolocation. The elemental can't use its blindsight while deafened.

Hold Breath. The elemental can hold its breath for 30 days.

Keen Hearing. The elemental has advantage on Wisdom (Perception) checks that rely on hearing.

Siege Monster. The elemental deals double damage to objects and structures.

Legendary Resistance (2/Day). If the whale fails a saving throw, it can choose to succeed instead.

Multiattack. The elemental either makes a Bite attack or uses its Swallow, and then makes a Tail attack.

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 19 (3d6 + 9) slashing damage. If the target is a creature, the target is grappled. Until this grapple ends, the target is restrained, and the elemental can’t bite another target.

Swallow. The elemental makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the elemental, and it must make a DC 19 Constitution saving throw or take one level of exhaustion on a failed save, or take 7 (2d6) cold damage on a successful save, at the start of each of the elemental’s turns.

If the elemental takes 50 damage or more on a single turn from a creature inside it, the elemental must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the elemental. If the elemental dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Tail. Melee Weapon Attack: +16 to hit, reach 5 ft., one target. Hit: 29 (5d8 + 9) bludgeoning damage.

Cold Geyser (Recharge 5–6). The elemental blasts frigid air in a 40-foot cone. Each creature in that cone must make a DC 19 Constitution saving throw, taking 66 (12d10) cold damage on a failed save, or half as much damage on a successful one.

The elemental can take 2 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 elemental regains spent legendary actions at the start of its turn.

Tail Attack. The elemental makes one Tail attack, and then moves up to half its swimming speed.

Exhale (Costs 2 Actions). The elemental uses its Cold Breath if it’s charged.