Swarm of Insects

A swarm of insects on the party's side is the druid's wild shape pull, the necromancer-adjacent witch's bound familiar, or the result of a Conjure Animals at the right CR. Most often it is the druid: a level 4 druid wild-shaping into a Medium swarm of bees or biting flies, then walking into the room the party can't enter. Frame it that way and the table understands immediately.

In combat the swarm's contribution is condition immunity and shape. It walks through narrow gaps the party can't, occupies the same square as an enemy spellcaster (forcing concentration checks every turn), and ignores everything the enemy tries to do to lock it down. No grapple, no fear, no hold. The damage output is low, but the swarm doesn't need to deal damage to win. It needs to camp on the lich's square and break every spell the lich tries to cast.

Outside combat the swarm is reconnaissance through impossibly small openings. A swarm of bees hovers outside a window and listens; a swarm of beetles crawls under a door and reports back what it touched. The 20 ft. fly speed (if the GM picked that variant) is slow but unstoppable in any building with vents.

The swarm dies fast to fire. A torch-wielding guard ends the bit. Plan the entrance and exit so the party doesn't waste their best tool on a corridor they could have walked through.

A swarm of insects is a damage-resistance puzzle wearing a body. The SRD entry here is sparse on actions, so the encounter math is structural: AC 11, 19 HP, resistance to Bludgeoning, Piercing, and Slashing, immunity to Charmed, Frightened, Grappled, Paralyzed, Petrified, Prone, Restrained, and Stunned. Almost every condition the party uses to lock down a single enemy is gone. The fighter who built around grappling is useless. The monk who stuns is useless. The wizard who casts Hold Person watches the spell fail before it starts.

Run the swarm as terrain. The 20 ft. walk speed plus optional 20 ft. climb or fly (your choice at session start) means a single swarm should be impossible to outrun in a confined space and trivial to outrun on open ground. Pick the venue accordingly. A cellar, a sewer, a tomb corridor, anywhere the party can't simply walk away. The swarm enters a PC's square (it can do this because it is a swarm), and now they are sharing space with biting insects, can't easily move without taking the swarm with them, and can't condition-lock it off.

Damage priority is whoever the swarm last touched. Without a Multiattack or special action in this entry, run the swarm as a generic Tiny-beast swarm bite at +3 for around 4 to 5 piercing or poison damage to anyone in its space, dropping by half once Bloodied. The point of the encounter is not the damage. The point is the spell components dropped because the wizard is brushing wasps off her face. Concentration checks should come up every round.

Fire and area damage end the encounter. The first Burning Hands or Thunderwave kills the swarm in one round. That is fine. The swarm's job is to occupy two rounds before the party finds the right tool, not to win.