Woshar Baaz's Absolute Incineration

This spell creates an area of extreme heat that incinerates creatures into nothing but ash. Roll 12d8; the total is how many hit points of creatures this spell can affect. Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points.

Starting with the creature that has the lowest current hit points, each creature affected by this spell dies and turns to ash. Subtract each creature's hit points from the total before moving on to the creature with the next lowest hit points. A creature's hit points must be equal to or less than the remaining total for that creature to be affected. If a creature is Resistant or immune to fire damage, they are skipped over by this effect

Creatures in the area of effect that aren't affected by the incineration all take 12d6 fire damage.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, roll an additional 2d8/2d6 for each slot level above 5th.