Amphibious.

The riverine can breathe air and water.

Legendary Resistance (3/Day).

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

Actions

Multiattack.

The riverine makes two Flood Strike attacks.

Flood Strike.

Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage plus 10 (3d6) cold damage.

Spellcasting.

The riverine casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 17):

At will: control water, fog cloud
1/day: greater restoration

Bonus Actions

Whirlpool Step.

The riverine magically teleports to an unoccupied space it can see within 30 feet of itself. Both the space it leaves and its destination must be in or on the surface of water.

Legendary Actions

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

Whirlpool Rush. The riverine uses its Whirlpool Step. Immediately after it teleports, each creature within 5 feet of the riverine's destination space takes 5 (1d10) cold damage.
Raging Deluge (Costs 2 Actions). The riverine unleashes a torrent of river water in a 30-foot line that is 5 feet wide. Each creature in that area must make a DC 17 Dexterity saving throw. On a failed save, a creature takes 11 (2d10) bludgeoning damage and is knocked prone. On a successful save, a creature takes half as much damage and isn't knocked prone.

Lair Actions

On initiative count 20 (losing initiative ties), the riverine can take one of the following lair actions; the riverine can't take the same lair action two rounds in a row:

Phantasmal Mist[–]

The riverine creates fog around a creature it can see within 120 feet of it. The target must succeed on a DC 17 Wisdom saving throw or take 5 (1d10) psychic damage and be frightened of the riverine until the end of the target's next turn.
River's Fury[–]

The riverine conjures swampy ground that briefly covers the ground in a 20-foot square the riverine can see within 120 feet of itself. That area becomes difficult terrain until initiative count 20 on the next round. Any creature, other than the riverine, that enters the affected area or starts its turn there takes 7 (2d6) cold damage.

Regional Effects

The region containing a riverine's lair flourishes under the magic of the riverine's presence, which creates one or more of the following effects:

Clearwater Sight. The waters of the river serve as a conduit for the riverine's consciousness. As an action, the riverine can cast the clairvoyance spell, requiring no spell components, targeting any point along the river it's connected to.
River's Bounty. Freshwater fauna and flora reproduce rapidly and thrive in the waters within 1 mile of the riverine's lair. Foraging in these waters yields twice the usual amount of food.
Spirit Guidance. Tracks appear along the banks of the river within 6 miles of the riverine's lair. The tracks lead to potential shelters and safe passages across the river's waters, while also leading away from areas that the riverine prefers to remain undisturbed.

If the riverine dies, the flora and fauna return to normal levels, and its river dries out over the course of 1d10 days. All other effects cease immediately.

Riverine