Swarm of Piranhas

A swarm of piranhas as ally is a stretch, but the framings exist. A druid's Wild Shape variant in a homebrew setting that allows swarm forms. A river-witch the party has befriended sends her piranhas as a one-time favor against an enemy who must be lured into the water. The party owns a property with a moat that they've stocked deliberately as a defense system.

Mechanically the use case is precise: drive an enemy into the water and let the swarm do the work. The party's job is the drive. The swarm's job is the consequence. Out of combat the swarm has no utility except as a deterrent. NPCs who learn what's in the moat do not swim across the moat.

In a directed combat, the swarm Swims 40 ft. to a chosen body in the water and starts biting. INT 1 means it does not distinguish between targets the party has marked and targets the party hasn't, so anyone in the water with the swarm gets bitten. This is the rule. Tell the players. Do not let a fighter dive in to "help" without making sure they understand the swarm will not recognize them as a friend.

When the favor is done or the enemy is dead, the swarm disperses back into the river. There is no goodbye scene because there is no creature to say goodbye to. The river just goes quiet again, and the players remember that it didn't have to.

A swarm of piranhas is an environmental threat with a stat block. CR 1, AC 13, 28 HP, no listed actions, but the swarm rules and the situation do all the work. The encounter is "the river you have to cross is also the encounter," and the trick is making the players feel the pressure of water they thought was a backdrop.

Set up the encounter with the swarm already in the water. Walk 5 ft. and Swim 40 ft. tells you the piranhas are only dangerous to people who are wet. Trigger them with blood: a wounded PC fords the river, or the rogue gets shoved off a bridge during a separate fight. Rule a swarm bite at +5 for around 10 piercing per turn against any creature in the swarm's square (reducing as the swarm takes damage, per the standard swarm rule). The damage is not the threat. The threat is that the target can't fight back effectively while swimming and the rest of the party can't get to them without entering the water themselves.

Swarms of tiny beasts are immune to Charmed, Frightened, Grappled, Paralyzed, Petrified, Prone, Restrained, and Stunned, and resist Bludgeoning, Piercing, and Slashing. Tell the players this when they start asking, because the fighter's Sentinel and the rogue's Ensnaring Strike both do nothing here. Fire, cold, lightning, force, and acid all work normally. A single Burning Hands or Thunderwave clears the swarm fast.

The swarm does not retreat in any meaningful sense. INT 1, no morale. It eats until the food source leaves the water or the swarm dies. The encounter ends when the targeted PC gets out or when the party brings AOE damage to bear.

Have the swarm strip a corpse to bone in one round of narration before the party realizes what's in the water. Then make them roll initiative.