Waterwraith

Amorphous. The waterwraith can move through a space as narrow as 1 inch wide without squeezing but its rollers cannot.

Fear Aura. Any creature that starts its turn within 30 feet of the waterwraith must make a DC 17 Wisdom saving throw, unless the waterwraith is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creature's saving throw is successful, the creature is immune to the waterwraith's Fear Aura for the next 24 hours.

Forced Physicality. If the waterwraith is stunned or petrified, it loses its immunity to bludgeoning, piercing, and slashing from nonmagical attacks until it is no longer stunned or petrified.

Freedom of Movement. The waterwraith ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. If the waterwraith starts its turn grappled, petrified, restrained, or stunned, it can reduce its speed by 5 feet until the end of its turn to end the condition. If it is under multiple instances of conditions, it must reduce its speed by 5 feet for each instance it ends. It cannot reduce its speed to below 0 feet.

Frenzy (Recharges after a Short or Long Rest). If the waterwraith is reduced to 0 hit points or both of its rollers are reduced to 0 hit points, the waterwraith immediately regains 150 hit points and its rollers are destroyed if they were not already. For one hour after activating this trait, the waterwraith can take Mythic Action options as legendary actions. Defeating the waterwraith after it uses this trait counts as defeating two CR 10 creatures.

Rollers. The waterwraith carries two rollers which it uses for ambulation and offense. Each roller is a 300 pound Large object with an Armor Class of 20 and 75 hit points and is immune to all damage except for thunder damage. While carrying its rollers, the waterwraith has advantage on saving throws to prevent or end the frightened condition. At the end of a short or long rest, the waterwraith can create a new pair of rollers which causes any existing rollers to be destroyed.

Unusual Nature. The waterwraith does not need to eat, drink, sleep, or breathe.

Pseudopod. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 12 (2d6 + 5) acid damage.

Crushing Roll. The waterwraith moves up to its speed in a straight line. While doing so, it can enter Large or smaller creatures' spaces. The first time the waterwraith enters a creature's space as part of this movement, the creature must make a DC 17 Dexterity saving throw.

On a successful save, the creature can choose to be pushed 5 feet to the side of the waterwraith. A creature that chooses not to be pushed or has no space to be pushed into suffers the consequences of a failed saving throw.

On a failed save, the waterwraith enters the creature's space, and the creature takes 21 (6d6) bludgeoning damage and is knocked prone. The waterwraith can take this action only while carrying its rollers.

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

Attack. The waterwraith makes a pseudopod attack.

Flatten (Costs 2 Actions). The waterwraith uses its Crushing Roll.

If the waterwraith’s Frenzy trait is active, it can use the Mythic Action options below as legendary actions.

Frenzied Attack. The waterwraith makes two psuedopod attacks.

Move. The waterwraith moves up to its speed.