Swarm of Rats

A swarm of rats as ally is a stretch and you should name it openly. The most plausible setups: a wererat NPC lends the party her swarm for one night, a beast-bond druid has wild-shaped a colony for a sewer mission, a necromancer has reanimated rats and given the party the command word for one favor. In all three cases the swarm is on a clock and on a leash, and the party should know what both look like.

Use the swarm as access. Walk 30 ft. and climb 30 ft. means a swarm squeezes through grates, gaps under doors, ventilation shafts, and storm drains the party cannot. Send the swarm under a door into a guarded room, let the controlling NPC borrow their senses to scout the chamber, then pull them back out before the guards notice. That is the play. It is not a combat play.

In a fight, the swarm is fragile (14 HP, AC 10) but also untargetable by the spells the enemy probably wants to use on it. Frightened, Charmed, Stunned, and Paralyzed all bounce off. The swarm's job in combat is to wash over an enemy spellcaster and force concentration checks every time the enemy takes a turn standing in it. One swarm on a wizard for two rounds is often two failed concentration saves, which is the entire reason the party borrowed the rats.

The relationship ends when the controlling NPC takes them back. The party should never own this swarm. They are renting it.

A swarm of rats is the basement encounter, the sewer encounter, the prison cell encounter, the granary-on-fire encounter. AC 10, 14 HP, walk 30 ft., climb 30 ft., resistance to bludgeoning, piercing, and slashing, immunity to charmed, frightened, grappled, paralyzed, petrified, prone, restrained, and stunned. The Bites action deals 2d4 piercing damage, or 1d4 if bloodied. The swarm cannot be controlled and cannot be knocked down. It just keeps coming.

Run multiple swarms. One swarm is a speed bump. Three swarms boiling out of three separate holes in a tenement basement is a real fight at level 1 and a memorable problem at level 3. Use the climb 30 ft. to get them onto the cleric's back from a rafter, into the rogue's cell from the wall, across the ceiling above the wizard's head. The swarm should never approach in a straight line. It comes from where the players were not looking.

The damage type is the pressure the players need to find. Swarms resist bludgeoning, piercing, and slashing, so a fighter swinging a longsword feels like he is hitting fog. Fire, cold, lightning, thunder, and acid work normally. The first time a torch dropped on a swarm clears it in one round, the table learns the rule. Every swarm encounter after that is a question of who has the firebomb. If nobody does, the encounter is harder than the CR suggests.

Pair the swarm with the environment. Rats in a flooded basement mean you cannot drop a fireball without also frying the wet rogue. Rats on a wooden granary floor mean a torch starts a structure fire. Rats in pitch darkness mean Darkvision matters and torchlight matters more. The swarm is a pressure on resources, not a damage check.

Describe the noise. The squeaking, the wet movement, the chittering on the stairs above.