Magma Mephit

A magma mephit on the party's side belongs to a fire genasi sorcerer, a dwarven forge cult, an efreeti's contracted servant pool, or a wizard who keeps a clutch in a brass cage as living thermometers and laboratory hazards. Mephits are Neutral Evil with Int 7, so they're petty, sarcastic, and cheaply bribed. The handler is the actual NPC, and the mephit is the pet that talks back.

In a fight, the mephit is a flying nuisance that the party deploys at a problem. Send it to scout a hot location it can't be hurt in (lava tube, forge, alchemist's lab). In combat, the Fire Breath is the only ability that matters: 15-foot cone, DC 11, 2d6 fire on a bonus to mid-tier monsters. Aim it through enemy clusters and accept that the recharge is brutal. Claw is filler. Don't keep the mephit in melee with anything that has multiattack, since 18 HP at AC 11 disappears against a single bandit captain.

The Death Burst is a problem. If the mephit dies adjacent to a party member, that's 2d6 fire on the friendly. The fire genasi doesn't care; the bard does. Position accordingly. When the deal expires (a daily contract, a single-job binding, the genasi sorcerer running out of pact tokens), the mephit insults the party in Primordial and flies off.

Give the mephit a name like a kitchen appliance and a one-line grievance. "Cinder hates being called Cinder" is more memorable than its DC 11.

A magma mephit is a CR 1/2 mook with a punchline. The party sees 18 HP, AC 11, and a small annoying elemental, and starts swinging. Then the mephit dies and Death Burst goes off for 2d6 fire on a DC 11 Dex save in a 5-foot emanation, and suddenly the wizard who walked up to finish it with a shocking grasp is at half HP. Run them in groups of three or four so the burst chain catches the same cluster twice.

Open with Fire Breath if recharge cooperates. DC 11 Dex save in a 15-foot cone for 2d6 fire averages 7 on failure, which is a third of a level-2 PC's HP for a single bonus-action-tier ability. After it's spent, the mephit closes for Claw at +3 for 3 slashing plus 1d6 fire. The damage is small but the mephit's job isn't damage; it's denying the party clean positioning. With 30 ft. fly, mephits hover above the front line and pelt squishies behind it, daring melee PCs to leave the wizard exposed.

Position the mephits so a Death Burst chain is possible. Two mephits adjacent means killing one takes the other to half. Three in a triangle around a paladin means the paladin eats three bursts in one round if a Fireball clears the room. The party should learn to spread the killing blows out across separate turns, and the GM gets a teachable moment about elemental swarms.

Cold vulnerability is the kill switch. A frost mage drops these in one cast. If the party doesn't have cold available, they should at least know to spread out before murdering them. Mephits are not brave; once two of the four are down, the rest fly straight up and out, looking for a thermal vent to dive into. They will return with friends.

Telegraph the burst by having the first mephit explode at a distance during a previous scene. The party respects what they've already seen kill someone.