Swarmkeeper. The swarmkeeper has a swarm of imps. They typically have 2d4+2, but can have other amounts. These imps will try to help the swarm keeper, but it takes the swarmkeeper an action to give them specific directions. When the swarmkeeper dies, all the imps are let free and are able to act on their free will.

Hurl Flame. Ranged Weapon Attack: +6 to hit, range 60/120 ft., 1 target. Hit: 6 (1d6 + 3) fire damage.

Command Swarm. The swarmkeepers gains completecontrol of it's swarm for a round. During this time, it can choose what action they use, where they move, and everything else about the swarm.

Gather into the Swarm. When a creature the swarmkeeper can see dies within 40 feet of it, it can use it's reaction to add the creature to it's swarm. When it does this, the creature becomes an imp which is part of the swarm keepers swarm.