Piranha

A friendly piranha school is a stretch, and the only framing that sells is "we are pointing a problem at a different problem." A druid drops fish food in the river to lure the school onto an enemy's flank. A hag keeps a farm under a pier as a security system. The party doesn't befriend the school; they aim it.

Mechanically the school is a hazard the party turns the wrong way. The enemy shoved or pulled into piranha-infested water takes the same flat damage per round the party would. A grappled cultist held under takes 1d4 to 2d4 piercing per round and probably starts drowning besides.

The trick is delivery. The party needs a way to get the target wet (Bigby's Hand, Thorn Whip, a rope and a saved-up shove), and the school does the rest. The school is not a teammate. Piranhas have Int 1 and don't follow commands, so the druid or witch character has to keep the bait alive or the school disperses. Once the bait stops bleeding, the fish leave and the party is back to normal combat.

Show the boil of the water before the trick goes off. The enemy who hears teeth before they hit the river is the enemy who panics in character.

A single piranha is 1 HP and worthless. The encounter is the school. Run piranhas in groups of 15 to 30 and treat the encounter as an environmental hazard with stats, not a normal combat. The party fell into the river, or someone bled into the water, or the rope bridge gave out at the worst moment.

Mechanically, treat each piranha as a single-bite irritation. The stat block shows Bite at +5 doing 1 piercing, with advantage if the target is not at full HP. What matters is mass. Anyone in the water swimming through the school takes a flat 1d4 or 2d4 piercing per round depending on density, with no save or attack roll per fish. A PC trying to grab a sinking ally loses HP every round they linger. The party is on a clock measured in rounds before someone drops.

Blood accelerates the school. If a PC is below half HP in the water, every piranha in a 30-foot radius converges. Make this visible: the water boils, the surface chops, the bard sees teeth. Anyone trying to revive a downed ally is now in a feeding zone. Don't ask for athletic checks every six seconds. Ask once, set a stake, and let the dice resolve.

Piranhas don't flee. They eat until full or until the food leaves the water. Once the party is on a bank or in a boat, the school dissipates. Whoever lost a finger keeps the scar.

Cut to the surface every round so the party in the water can see the boat is just out of reach.