Flames of Surtur

When you hit a creature with a spell that does fire damage, you may use your reaction to bolster it's strength

You deal an additional 1d8 fire damage as a part of that spell. This effect's damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8).

If the target has immunity to fire damage, they are considered resistant for this attack.
If the target has resistance to fire damage, they lose their resistance for this attack.
If the target has neither resistance nor immunity to fire damage, they are considered vulnerable for this attack.

After you have used this ability make a DC 15 CON save or suffer one level of exhaustion.

Once you use this feature you cannot use it again until you finish a short or long rest.