Ancient River Dragon

Legendary Resistance (4/Day, or 5/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Tide Sovereign. The dragon cannot be moved against its will while in contact with water.

Aura of Storms. Any creature starting its turn within 30 ft. of the dragon takes 10 (3d6) lightning damage as crackling arcs of energy lash outward.

Mythic Spellcasting. The dragon is a 15th-level spellcaster (Constitution-based, save DC 26). It can innately cast, requiring no components:

• At will: control water, shape water

• 3/day each: maelstrom, chain lightning, tsunami

• 1/day each: storm of vengeance, imprisonment (watery tomb)

Multiattack. The dragon makes three attacks: one bite, one claw, and one tail.

Bite. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 47 (6d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 33 (4d8 + 10) slashing damage.

Tail. Melee Weapon Attack: +18 to hit, reach 30 ft., one target. Hit: 39 (5d10 + 10) bludgeoning damage.

Breath (Recharge 5–6).

Freezing Torrent. The dragon exhales a gigantic line of hyper-pressurized freezing water 120 ft. long and 20 ft. wide. Each creature in the line must make a DC 24 Dexterity saving throw, taking 144 (32d8) cold and piercing damage on a failed save, or half as much on success. Creatures of Huge size or smaller are also knocked back 80 ft.

Lightning Paralysis Surge. The dragon exhales a cone of water and lightning in a 90 ft. cone. Each creature must make a DC 24 Constitution save. On a failed save, it takes 126 (28d8) lightning and bludgeoning damage and is paralyzed for 1 minute as lightning locks their muscles. The creature can repeat the saving throw at the end of each of its turns, ending the effect on a success.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

• Detect. The dragon makes a Wisdom (Perception) check.

• Tail Slam. The dragon makes a tail attack.

• Tidal Wave (Costs 2 Actions). The dragon summons a massive wave in a 60 ft. radius. Each creature must succeed on a DC 24 Strength save or take 31 (7d8) bludgeoning damage and be pushed 40 ft.

• Lightning Surge (Costs 3 Actions). The dragon releases a pulse of lightning in a 30 ft. radius. Each enemy must succeed on a DC 24 Constitution save or be stunned until the end of their next turn.