Durnash's Flames (5e Spell)
From D&D Wiki
|Casting time:||1 reaction, which you take when a creature within 60 feet of you that you can see takes damage|
The creature that took damage is wracked with flames of intensity proportional to its pain. The target must make a Constitution saving throw. It takes fire damage equal to the triggering damage on a failed save, or half as much damage on a successful one.