Swarm of Bats

A swarm of bats on the party's side is a creative stretch and a great one when it lands. A vampire NPC lends her swarm for one night. A druid wild-shapes into echolocation flight. A Conjure Animals scroll fills the room with bats at the moment the party badly needs obscurement. Frame the loan as temporary with a clear end condition.

The swarm's job is what it does to enemies, not what it does for the party. Send it ahead into a guard room. The guards have heavy obscurement, the party walks in behind it with advantage on the surprise round, and the rogue picks targets the swarm has helpfully blinded. Blindsight 60 means the swarm navigates corridors without light as a passable scout. If the swarm stops at a doorway and disperses on its own, something inside scared bats, and bats are not easily scared.

The swarm has 11 HP, so it does not survive a real fight. Use it once for a single push and accept that it will be gone in two rounds of serious damage. Resistances to bludgeoning, piercing, and slashing keep it alive against bandits, but a fireball ends the lend in one cast. Plan around that. The swarm is not a long-term companion. It is a one-scene asset.

When the loan ends, the bats fly back to wherever they came from, usually through a chimney at sunset in a swirling line the party will remember. Describe the silence of the room after the wings stop.

A swarm of bats is atmosphere with hit points. The stat block gives you 11 HP, AC 12, fly 30 ft., Blindsight 60 ft., and resistances to bludgeoning, piercing, and slashing damage. Immunities to charmed, frightened, grappled, paralyzed, petrified, prone, restrained, and stunned. The Bites action deals 2d4 piercing damage, or 1d4 if the swarm is bloodied. The encounter is not a fight in the conventional sense. It is a flying obscurement cloud and a flavor cue that something worse is about to be flushed out.

Run the swarm as a hazard layered over another encounter. Initiative rolls, the swarm fills a 10-foot space around the front-line PC, and now everyone in the cloud has heavy obscurement: attacks have disadvantage, and ranged attackers cannot pick targets. The swarm itself sees fine via Blindsight 60. The vampire spawn the swarm was guarding closes through cover the party cannot see through. The cultist walks to the cleric. That is the encounter.

If the party fights the swarm directly, AC 12 and 11 HP go down to almost any area of effect spell: Burning Hands, Thunderwave, or Shatter ends it in one cast. The party that swings martial weapons does half damage from resistances and wastes turns. Let them learn it once.

At half HP, the Swarm trait means the swarm does not panic but begins to disperse. Bats peel off each round, the cloud thins, and the obscurement effect dies before the swarm does. Describe the cloud breaking apart rather than the bats dying. The party gets line of sight back at exactly the moment the larger threat is on top of them.

Stage the swarm as the bats coming out of the cave roof when the torches go up. The flap of wings is the encounter. The HP is paperwork.