Combustion

You place a fiery mark on a creature that you can see within range. Until the spell ends, you deal an extra 1d6 fire damage to the target whenever you hit it with an attack. If your attack deals fire damage to the target, the extra damage increases to 1d8 fire damage instead.

If the target drops to 0 hit points before this spell ends, you can use a bonus action on a subsequent turn of yours to mark a new creature.

You can recast the spell on a creature that already bears the mark to end the effect and make the creature explode. The target must must make a Constitution saving throw. The target takes 2d10 fire damage on a failed save, or half as much damage on a successful one.

At Higher Levels. When you recast this spell on a target already bearing the mark using a spell slot of 2nd level or higher, the explosion damage increases by 1d10 for each slot level above 2nd. There is no change in the initial extra damage if you cast the spell on higher levels.