Ancient Water Elemental (Olhydra)

Empowered Attacks. The elemental’s slam attacks are treated as magical for the purpose of bypassing resistance and immunity to nonmagical weapons.

Innate Spellcasting. The elemental's innate spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: wall of ice

3/day: ice storm

1/day: storm of vengeance

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

Magic Resistance. The elemental has advantage on saving throws against spells and other magical effects.

Water Form. The elemental can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

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

Multiattack. The elemental makes two slam attacks or two water jet attacks.

Slam. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage, and the target is grappled (escape DC 19). The elemental can grapple up to four targets. When the elemental moves, all creatures it is grappling move with it.

Water Jet. Ranged Weapon Attack: +12 to hit, range 120 ft., one target. Hit: 21 (6d6) bludgeoning damage, and the target is knocked prone if it fails a DC 19 Strength saving throw.

Summon Elementals (1/day). The elemental summons up to three water elementals and loses 30 hit points for each elemental it summons. Summoned elementals have maximum hit points, appear within 100 feet of the elemental, and disappear if the elemental is reduced to 0 hit points.

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

Crush. One creature that the elemental is grappling is crushed for 21 (3d10 + 5) bludgeoning damage.

Fling (Costs 2 Actions). The elemental releases one creature it is grappling by flinging the creature up to 60 feet away from it, in a direction of its choice. If the flung creature comes in contact with a solid surface, such as a wall or floor, the creature takes 1d6 bludgeoning damage for every 10 feet it was flung.

Water to Acid (Costs 3 Actions). The elemental transforms its watery body into acid. This effect lasts until the elemental’s next turn. Any creature that comes into contact with the elemental or hits it with a melee attack while standing within 5 feet of it takes 11 (2d10) acid damage. Any creature grappled by the elemental takes 22 (4d10) acid damage at the start of its turn.