Conflugration

You strike the match, creating one cubic foot of Elemental Flame. As long as this flame burns, you are immune to Fire damage. Elemental Flame burns all solids and liquids, with the exception of Elemental Water, Elemental Earth, and materials native to the plane of Fire. It spreads one foot in all directions into any susceptible material at initiative count 20 each turn. This flame destroys anything it burns, not even leaving behind ashes. 

Creatures and objects that are burning from Elemental Flame take the damage at the start of each of their turns. Object turns are on initiative count 20. The first turn they are burning, the objects and creatures take damage as if they were a Fine creature. If they are larger, their next turn deals damage as if they were a Diminutive creature. This increase continues until their size is met. The creature continues burning until they die. Their body is burned on their next turn. Any worn or carried equipment is destroyed at this time as well. If an object that is burning is significantly larger than the below sizes (e.g. oceans, planets, planes, etc.) the DM will determine the speed at which it burns as to best serve the narrative. For reference, Earth would reasonably be fully consumed and destroyed in 8 years.

This flame continues to burn in place regardless of the precense of fuel, for all eternity. It can only be extinguished with the Wish spell, intervention by a God of Fire, or Elemental Water. It cannot cross through planar boundaries or portals. It can destroy magic items as if they were unenchanted. 

To pass through a space in which this fire is burning, without lighting yourself on fire, make a Dexterity Saving Throw. On a failure, you are ignited.

Upon casting this spell, the caster's soul is immediately engulfed in Elemental Flame, visible on the Ethereal Plane. Being immune to fire, they are unharmed. The flame upon the caster's soul is never extinguished. If the spell ends, the caster's soul continues to burn. If a suitable intervention (such as immunity granted by a deity) is not administered, the caster's soul is destroyed and they die instantly, incapable of being revived by any means short of a Fates card or unique Artifact.

Maximum Damage per Size Category

Fine (0.5 feet) - 1d10

Diminutive (1 foot) - 2d10

Tiny (2.5 feet) - 4d10

Small (5 feet) - 8d10

Medium (5 feet) - 16d10

Large (10 feet) - 20d10

Huge (15 feet) - 30d10

Gargantuan  (20 feet)- 35d10

Colossal (25 feet)- 40d10

Colossal+ (30+ feet)- 50d10