XGE has a spell that lets anything that breaths spew magical energy from its mouth for 10 rounds called Dragon's Breath (p. 154). Part of it reads:
Each creature in that area must make a Dexterity saving throw, taking 3d6 damage of the chosen type on a failed save, or half as much damage on a successful one.
It's been established that you can put it on your familiar, per Can a familiar from the Find Familiar spell use Dragon's Breath?
Do you use your own spell save DC or your familiar's? What would a familiar's spell save DC be? Seems like if it's a Dexterity save, it should be coming from the source of the fire.