Amphibious.

The giant can breathe air and water.

Legendary Resistance (1/Day).

If the giant fails a saving throw, it can choose to succeed instead.

Actions

Multiattack.

The giant makes two Lightning Sword attacks, or it uses Wind Javelin twice.

Lightning Sword.

Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 40 (9d6 + 9) lightning damage.

Wind Javelin.

The giant coalesces wind into a javelin-like form and hurls it at a creature it can see within 600 feet of it. The javelin deals 19 (3d6 + 9) force damage to the target, striking unerringly. The javelin disappears after it hits.

Legendary Actions

The storm giant quintessent can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The storm giant quintessent regains spent legendary actions at the start of its turn.

Gust. The giant targets a creature it can see within 60 feet of it and creates a magical gust of wind around the target, who must succeed on a DC 18 Strength saving throw or be moved up to 20 feet in any horizontal direction the giant chooses.
Thunderbolt (Costs 2 Actions). The giant hurls a thunderbolt at a creature it can see within 600 feet of it. The target must make a DC 18 Dexterity saving throw, taking 22 (4d10) thunder damage on a failed save, or half as much damage on a successful one.
One with the Storm (Costs 3 Actions). The giant vanishes, dispersing itself into the storm surrounding its lair. The giant can end this effect at the start of any of its turns, becoming a giant once more and appearing in any location it chooses within its lair. While dispersed, the giant can't take any actions other than lair actions, and it can't be targeted by attacks, spells, or other effects. The giant can't use this ability outside its lair, nor can it use this ability if another creature is using a control weather spell or similar magic to quell the storm.

Lair Actions

A storm giant quintessent can use lair actions in giant form and while transformed into a storm. On initiative count 20 (losing initiative ties), the giant can take one of the following lair actions; the giant can't take the same lair action two rounds in a row:

Deafening Boom. The giant creates a thunderclap centered on a point anywhere in their lair. Each creature within 20 feet of that point must succeed on a DC 18 Constitution saving throw or be deafened until the end of its next turn.
Fog. The giant creates a 20-foot-radius sphere of fog (or murky water if within water) centered on a point anywhere in its lair. The sphere spreads around corners, and its area is heavily obscured. The fog lasts until the giant disperses it (no action required), and it can't be dispersed by wind.
Gale. The giant creates a 60-foot-long, 10-foot-wide line of strong wind (or strong current within water) originating from a point anywhere in its lair. Each creature in that line must succeed on a DC 18 Strength saving throw or be pushed 15 feet in the direction the wind is blowing. The gust disperses gas or vapor, and it extinguishes candles, torches, and similar unprotected flames in its area. Protected flames, such as those of lanterns, have a 50 percent chance of being extinguished.

Regional Effects

The region containing a storm giant quintessent's lair is warped by the giant's presence, which creates one or more of the following effects:

Downpour. Rain, snow, or blowing dust or sand (whichever is most appropriate) is constant within 1 mile of the lair. Rain causes rivers and streams to fill or overflow their banks; snow, dust, or sand forms deep drifts or dunes.
Lightning. Flashes of lightning and peals of thunder are continual, day and night, within 5 miles of the lair.
Winds. High wind blows within 1 mile of the lair, making it impossible to light a fire unless the location where the fire is lit is protected from the wind.

If the giant dies, the lightning, thunder, and high wind regional effects end immediately. Rain, snow, and blowing dust abate gradually within 1d8 days.

Environment: Coastal, Desert, Mountain, Underwater

Storm Giant Quintessent