Combustion

You conjure an aura of pure fire around yourself. This aura is warm to the touch, but does not ignite objects. The next time you cast a spell that deals fire damage, all creatures affected by the spell gain vulnerability to fire damage. If they are resistant to fire damage, the spell ignores their resistance instead. If they are immune to fire damage, the spell treats their immnity as resistance instead, and the creature can't heal from the damage of the spell. If they are already vulnerable to fire, they take maximum damage instead.

When you cast this spell, you can cast a second spell of 1st level or higher on your turn, but only if it deals fire damage.

After the augmented spell deals its damage, this spell ends and the target's resistances, immunities, and vulnerabilities return to normal.

At Higher Levels. When you cast this spell using a spell slot of 9th level or higher, it lasts for an extra fire spell for every two levels above 7th.