Amatsu

Amphibious. The amatsu can breathe air and water

Calamitous Storm (Recharges after a Short or Long Rest). If the amatsu would be reduced to 0 hit points, its current hit point total instead resets to 300 hit points, and its Tail attack deals an additional 4 (1d8) lightning damage. Additionally, the amatsu can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 50,000 XP (100,000 XP total) for defeating the amatsu after its Calamitous Storm activates.

Heart of the Storm. The precipitation and wind levels in a 1 mile radius around the amatsu are increased by 2 stages each as shown in the tables of the control weather spell. If the amatsu dies, the weather returns to normal over the course of 1d10 minutes.

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

Multiattack. The amatsu makes one Bite attack and one Tail attack.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 30 (4d10 + 8) piercing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 26 (4d8 + 8) bludgeoning damage, and the target must succeed on a DC 23 Strength saving throw or be pushed back 5 feet and be knocked prone.

Typhoon Breath (Recharge 5–6). The amatsu spits out a ball of high-pressure water at a point it can see within 90 feet of it. The ball explodes in a 30-foot radius sphere. Each creature within the area must make a DC 23 Dexterity saving throw, taking 33 (6d10) bludgeoning damage plus 33 (6d10) acid damage on a failed save, or half as much damage on a successful one.

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

Tail Swing. The amatsu makes one Tail attack.

Wind Funnel (Costs 2 Actions). The amatsu creates a funnel of wind in a 60-foot long, 15-foot wide line. Each creature within the area must make a DC 23 Strength saving throw, taking 16 (3d10) bludgeoning damage plus 16 (3d10) thunder damage and be pushed up to 10 feet away horizontally on a failed save, or half as much damage and no additional effects on a successful one.

Tornado (Costs 3 Actions). The amatsu creates a 90-foot tall, 10-foot radius tornado at a point it can see within 90 feet of it that lasts until the start of the amatsu's next turn. Each creature that starts its turn within 60 feet of the tornado or moves to a space within 60 feet of the tornado must succeed on a DC 23 Strength saving throw or take 11 (2d10) thunder damage, and be pulled up to 30 feet horizontally toward the center of the tornado. A creature that starts its turn or moves into the tornado's area takes an additional 22 (4d10) bludgeoning damage, and is pulled up to 20 feet vertically toward the top of the tornado.

If the amatsu's Calamitous Storm trait has activated in the last hour, it can use the options below as legendary actions.

Lightning Strike. The amatsu calls down a bolt of lightning to strike a creature that it can see within 120 feet of it. The creature must succeed on a DC 23 Dexterity saving throw or take 22 (5d8) lightning damage.

Typhoon Blast (Costs 3 Actions). The amatsu recharges its Typhoon Breath and uses it.