Black Flame Firebolt

You hurl a mote of pitch black flame at a creature or object within range. Make a ranged spell attack against the target. On a hit, the target takes 1d8 fire damage, and an additional 1d4 fire damage for every 25 health the enemy has. This damage ignores resistance to fire damage, and treats immunity like resistance. A flammable object hit by this spell ignites if it isn't being worn or carried. 

this spell does not count against the maximum number of known cantrips, and cannot be learned through standard leveling means. 

The damage from this spell increases by 1d8 at levels 5, 11, and 17