Desolation

You start amassing three gouts of flame into a large congealed mass around and above you, each gout of flames is a 5-foot cube wide and appears within 15ft of you connecting to the sphere around you, the flame sphere is a 20ft sphere centred on the caster. If a creature gets within 5ft of either the gouts of flame or the sphere, they immediately take 6d10 fire damage and any flammable objects on them are immediately set ablaze. Each round after the first any creatures within the area take 6d10 fire damage, until the spells completion the area of effect steadily grows by 5ft, sand heats and starts to become glass and eventually that glass melts away as well, trees burn into dark husks, any other vegetation is burnt to cinders and any water is evaporated. 

Upon completion of the spell the gouts of flame burn off and the sphere rises above your head, at this time you may choose to activate the spell or attempt to extinguish the flames.

If you choose the try and extinguish the flames you must make a Wisdom saving throw of your spell save DC + half of your DC. On a successful save, you extinguish the flames and suffer 5 points of exhaustion from the strain of the spell. On a failed save, the magic holding the sphere spikes and tries to retreat back to you, the flames engulf you dealing 40d8 fire damage to you as the flames try hold together, if you are dropped to 0 hit points your body is left a desolate husk with a concentrated mass of flames where you heart would be, exuding intense heat which prevents any plant-life magical and non-magical to grow within 30ft of you and any creature that gets close to suffer 6d10 fire damage. 

If you choose to activate the spell, you detonate the sphere causing all creature's within a mile's radius of you to make a Constitution saving throw or take 40d8 fire damage, half as much on a successful save. A creature reduced to 0 hit points is disintegrated from the heat and can only be brought back to life by no less than a Wish or True Resurrection.

If you are dropped to 0 hit points your body is left a desolate husk with a concentrated mass of flames where you heart would be, exuding intense heat which prevents any plant-life magical and non-magical to grow within 30ft of you and any creature that gets close to suffer 6d10 fire damage.