5e SRD:Water Elemental
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Large elemental, neutral
Damage Resistances acid; bludgeoning, piercing, and slashing from nonmagical attacks
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.
Freeze. If the elemental takes cold damage, it partially freezes; its speed is reduced by 20 feet until the end of its next turn.
Multiattack. The elemental makes two slam attacks.
Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.
Whelm (Recharge 4-6). Each creature in the elemental's space must make a DC 15 Strength saving throw. On a failure, a target takes 13 (2d8 + 4) bludgeoning damage. If it is Large or smaller, it is also grappled (escape DC 14). Until this grapple ends, the target is restrained and unable to breathe unless it can breathe water. If the saving throw is successful, the target is pushed out of the elemental's space.
A water elemental is a cresting wave that rolls across the ground, becoming nearly invisible as it courses through a larger body of water. It engulfs creatures that stand against it, filling their mouths and lungs as easily as it smothers flame.