Killer Whale

An allied killer whale is a bonded mount or a druid's wild shape. The cleanest framing is a coastal village's tradition: a single orca raised from a calf with a particular family, allowed to leave at any time, returns when called by a specific bone whistle. Other workable framings include a triton ranger who treats the whale as a peer, a sea hag's gift the party hasn't yet realized is a leash, or a Tideborn campaign where the orcas and the surface coalition have a working pact.

In the water it carries one Medium PC on its back at 60 ft. of swim, which is genuinely transformative for an underwater dungeon or a long open-sea pursuit. Blindsight 120 ft. means the rider has a tactical map of the area without making any checks. In a fight, the whale rams enemies (improvise from Strength 19, around +6 to hit, 3d8 bludgeoning plus a DC 14 Strength save or prone) and bites once per round for 5d6+4 piercing. It will not chase enemies onto land. Walking speed is 5 ft. and the moment the deck is dry the whale is done.

Out of combat, the whale is the campaign's logistics solution: it carries cargo, scouts reefs, and hauls a swamped boat home. The price is feeding it, which is significant. A Huge beast eats a hundred pounds of fish a day, and a coastal town will notice when the party's pet starts working through the local cod.

Have the whale answer to a name only one PC can pronounce correctly. The whale will surface for that PC and ignore the rest until it is in the mood.

A killer whale encounter is an environmental fight, not a melee fight. The party is in the water or on a small boat, and the whale is doing the choosing. Walking speed is 5 ft., swim is 60 ft., and Blindsight 120 ft. means the whale has a perfect read on every PC in the water from two rooms away. The party is in its house.

The SRD entry doesn't list specific actions, so improvise within the stat line. Use a Bite (around +6 to hit, 5d6+4 piercing) and a ramming charge that swims 30 ft. in a straight line and forces a DC 14 Strength save or the target is knocked prone in the water and takes 3d8 bludgeoning. Multiattack is two of those, usually one bite and one ram. Open with the ram against the heaviest-armored PC, since plate armor in water is already a death sentence and the whale is happy to help.

The encounter shape is push and chase. The whale rams a target, retreats to 60 ft., circles, and comes back next round. AC 12 and 90 HP means a focused barrage from a longbow ranger or an Eldritch Blast warlock can punish it from the deck of a boat, so the whale dives when it gets bloodied and surfaces elsewhere. Stealth +4 underwater plus Blindsight makes the next charge a surprise even though the party knows it's coming.

It will not fight to the death. A pod-mate showing up wounded, or one PC dragged below the surface, gives the whale a reason to leave with its meal. It is not angry. It is hungry, and the party is the easiest prey on the surface for miles.

Stage the fight at twilight with the boat already low in the water. The first sign should be the wrong shape under the hull, then the boat tilts, then someone is gone.