Swarm of Insects

A swarm of insects on the party's side is the druid's wild shape pull, the witch's bound familiar, or the result of a Conjure Animals at the right CR. Most often it is the druid at level 4 wild-shaping into a Medium swarm of bees or biting flies, then walking into the room the party cannot 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 cannot, 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 does not need 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 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 moment. Plan the entrance and exit so the party does not waste their best tool on a corridor they could have walked through.

A swarm of insects is a damage-resistance puzzle wearing a body. 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.

The swarm has walk 20 ft., climb 20 ft., and fly 20 ft. (you pick which of climb or fly at session start based on the location). Bites action deals 2d4+1 poison damage, or 1d4+1 if bloodied. Run the swarm as terrain. In a confined space like a cellar or sewer, a single swarm should be impossible to outrun. In open ground, it is trivial. Pick the venue accordingly.

The swarm enters a PC's square (Swarm trait allows this), and now they are sharing space with biting insects, cannot easily move without taking the swarm with them, and cannot condition-lock it off. Damage priority is whoever the swarm touches. The point of the encounter is not the damage. The point is the spell components dropped because the wizard is brushing insects 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.