Water Elemental

A water elemental ally is bound to a druid's circle, summoned for a specific task by a cleric of a sea god, or owed to the party by a marid who lost a bet. The elemental has no loyalty in the social sense; it serves the contract until the contract ends, and it returns to the Plane of Water the moment the conditions are met. Set those conditions in front of the players and stick to them.

In a fight near water, the elemental is the encounter. Position it submerged and let it Whelm enemies as they wade in: DC 15 Strength save, 22 average plus the suffocation count starts. The party fights from the bank or the boat while the elemental drags the enemy frontline into the depths one by one. Water Form lets the elemental flow through hull breaches, sewer grates, and any narrow gap, so it can deliver the party into spaces other allies cannot reach. Slam is the cleanup tool for anything that stays out of the water.

Out of water, the elemental loses most of its identity. The 30 ft. walk speed is slow, Whelm without water is much less dangerous because the suffocation rule depends on the target being submerged, and the elemental cannot hide. Avoid these scenes. If the party needs the elemental to come along on a desert leg, the contract should end at the city gate.

Resistance to fire and acid plus immunity to poison means the elemental tanks for the party against creatures that would shred them: a fire snake, a black dragon's breath, a giant's poisoned spear. Use this when the party knows the threat in advance and asks for the elemental specifically.

Have the elemental speak only in Aquan, only through one party member who learned the language for this. Players love a translation problem that costs them nothing but spotlight.

A water elemental is a drowning hazard with a stat block. The whole encounter changes based on whether there's water nearby, and you should design the room before you roll initiative.

Open in the water if you can. Swim 90 ft. is triple the elemental's walk speed, and Water Form lets it slide through any space at least an inch wide, which means grates, cracks, and the gap under doors do not slow it. Whelm is the win condition: DC 15 Strength save, 4d8+4 bludgeoning, plus the target is Grappled, Restrained, suffocating if they can't breathe water, and takes 2d8 bludgeoning at the start of each of the elemental's turns. Recharge 4-6 means the elemental gets it most rounds. The elemental can hold one Large or two Medium creatures at a time, so prioritize the heaviest-armored melee character (because of the suffocation) and the cleric (because the suffocation interrupts concentration).

Slam is the round-filler. +7 to hit, 13 bludgeoning, and Medium-or-smaller targets are knocked Prone. Use Slam on whoever just stood up out of the water to deny the next round's actions. The condition immunity list is huge (Grappled, Paralyzed, Petrified, Prone, Restrained, Unconscious, Poisoned, Exhaustion), so most martial control just slides off. The party's answer is damage, and 114 HP behind AC 14 is real but reasonable for CR 5.

Freeze is the lever the smart party will find. Any cold damage drops the elemental's speed by 20 ft. until the end of its next turn, which lets the party disengage and fight from dry land. If the party has no cold, the elemental controls the engagement. It will not pursue onto dry ground; it either reforms in the nearest pool or dissipates and waits.

Run the suffocation as a count. "Round one Whelm. You can hold your breath for one minute." Then announce the round. The clock makes the rescue feel real.