Magmin

A friendly magmin is a novelty pet, almost always a wizard's bound companion or a souvenir from a planar visit gone sideways. The framing that works at the table is the magmin as a low-power utility creature: it lights rooms, melts locks slowly, scares horses, and occasionally explodes in someone's face. Establish the leash up front. The summoner has it for a fixed duration, or the magmin has been promised passage home if it serves for one tenday, or it is simply lost and following the party because they have not yet told it to leave.

In combat, the magmin is an opener. Send it forward, let it Touch the nearest enemy for 2d4+2 fire, and accept that the party's tank will catch a Death Burst if the magmin drops. Fire immunity means the party can drop a fireball on the magmin's position without worrying about the magmin, which is a real tool when the wizard wants a clean AOE on a clustered melee. Ignited Illumination is also useful as a bonus-action torch the party never has to relight.

Out of combat, the magmin is a hazard the party brought with them. It will set the inn carpet on fire if left unsupervised. The innkeeper does not care that it's chaotic neutral. Roleplay the cost: stables refuse to hold it, ferries charge double, the party's bard has to talk past at least one mayor a session.

When the duration ends, the magmin pops back to the Plane of Fire in a final burst. If the players grew attached, give them one warm beat to say goodbye, then a small thump of heat and the spot on the floor where it stood.

A magmin is a CR 1/2 firework. 13 HP, AC 14, and Death Burst means it explodes when killed for 2d6 fire on a DC 11 Dex save in a 10-foot emanation. Run them in groups of three to six and the encounter takes care of itself, because the party's first big AOE turns the front rank into a chain of overlapping bursts.

Open by closing the distance. Speed 30 and a Touch attack at +4 for 2d4+2 fire on hit means the magmin is in melee turn one. The bonus action Ignited Illumination is on by default in any fight, so each magmin is also a 10-foot radius of bright light, which kills the rogue's hide attempts and lets ranged enemies see across the room. Touch starts flammable objects burning, including a creature's clothing or pack if the GM is generous about ruling. The fire is small but it stacks across rounds.

Target priority is whoever stands closest. Magmins are not clever; Intelligence 8 and Wisdom 11 means they don't pick the right target, they pick the nearest one. Fire immunity means the party's pyromancer is wasting slots; the cleric needs to swing radiant or cold or just basic weapons. The real tactical question is positioning. A smart party will spread out to keep Death Burst from chaining; a dumb party will cluster, and the first magmin kill takes 2d6 off the next two PCs in line, who then trigger their own bursts when the chain wave hits the magmin behind them.

Magmins do not retreat. They were lit up by something (a fire elemental cult, an efreet's contract, a ruptured forge in the Plane of Fire) and they keep walking until they detonate. If you want a softer exit, tie them to a brazier or summoning circle the party can disrupt. Smash the brazier and the remaining magmins gutter out where they stand.