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 has lent her swarm for one night. A druid wild-shaped a moon shape into echolocation flight. A scroll of Conjure Animals filled the room with bats at a moment the party badly needed obscurement. Frame the loan as temporary, with a clear end condition, and play the gift as borrowed power.

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 itself navigates corridors without light, which makes it 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 fight. Use it once for a single push and accept that it will be gone in two rounds of any real 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 as the witch's gift coming home. Describe the silence of the room after the wings stop.

A swarm of bats is atmosphere with hit points. The SRD stat block lists no Bites or attacks, just the swarm chassis: 11 HP, AC 12, fly 30, Blindsight 60, resistant to physical damage, immune to most conditions. 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 of the cave.

Run the swarm as a hazard layered over another encounter. Initiative rolls, the swarm fills a 10-foot cube around the front-line PC, and now everyone in the cloud has heavy obscurement: attacks have disadvantage, and ranged attackers cannot pick targets cleanly. 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 with the dagger 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 AOE: a Burning Hands, a Thunderwave, a Shatter ends it in one cast. The party that swings martial weapons does half damage from the resistances and wastes turns. Let them learn it once.

Swarms do not panic and do not retreat in the way a single creature does. At half HP they begin to disperse: a few 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.