Swarm of Piranhas

A swarm of piranhas as ally is a stretch, but the framings exist. A druid's Wild Shape 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 they have stocked deliberately as a defense system.

Mechanically the use case is precise: drive an enemy into the water and let the swarm 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 is in the moat do not swim across it.

In directed combat, the swarm swims 40 ft. to a chosen body in the water and starts biting. Intelligence 1 means it does not distinguish between targets the party has marked and targets it has not, so anyone in the water with the swarm gets bitten. Tell the players before they dive in to "help." Do not let a fighter enter the water without understanding 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 did not have to.

A swarm of piranhas is an environmental threat with a stat block. CR 1, AC 13, 28 HP, swim 40 ft., Water Breathing. 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 creatures in the water. Trigger them with blood: a wounded PC fords the river, or the rogue gets shoved off a bridge. The Bites action gives +5 to hit with advantage if the target is not at full HP, dealing 2d4+3 piercing damage, or 1d4+3 if the swarm is bloodied. The damage is not the threat. The threat is that the target cannot fight back effectively while swimming and the rest of the party cannot reach 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. Fire, cold, lightning, force, and acid work normally. A single Burning Hands or Thunderwave clears the swarm fast.

The swarm does not retreat. Intelligence 1 means no morale. It eats until the targeted creature gets out or the party brings area damage to bear. Have the swarm strip a corpse to bone in one round of narration before the party realizes what is in the water. Then make them roll initiative.