Veladon's Entropic Furnace

You conjure fire from the essence of the multiverse, and shape it into an arrow over the course of casting the spell. 

casting period: When you start casting the spell, a 6 foot radius, 10 foot high cylinder of purple fire is conjured, centered beneath your feet. A pillar of purple tinted smoke rises to 9 times the fires height above the cylinder. Anyone in this cylinder but you takes 1 radiant damage at the end of their turn. At the start of each of your turns while casting the spell, this cylinder's radius increases by 1 foot, and it's height by 10. Once the casting is complete, you form an arrow from the fire, and can fire it as soon as the casting ends, or any of your turns following. Traditionally, the arrow is shot into the air, and descends on a point within 1,000 feet and detonates 250 feet above the ground facing directly downwards over that point, but it can be shot directly at anything within 1,500 feet. If shot directly, the spell deals ¼ its usual damage, and the caster takes 5d10 force damage from recoil of the unutilised flame as it dissipates.

Detonation: When fired, it detonates on your next turn. Upon detonation, two cylinders are filled with roaring purple flame, both centred on the base of the arrowhead. The first has a radius of up to 300 feet and is 250 feet tall, and pointing in the same direction as the arrow, downwards if the arrow is shot in it's traditional path. The second cylinder has the same radius to the first, and is 750 feet tall, and points the opposite direction of the arrow, upwards if the arrow is shot in it's traditional path.

Effect: Everything within either cone must make a constitution saving throw. It automatically fails this save if it is directly Infront of the arrow upon detonation. On a failed save, it takes damage equal to half its maximum hit points, or half as much on a successful one. This damage cannot exceed 100, but cannot be lower than 10. If a creature is resistant or immune to radiant, necrotic, or force damage, it takes 25 less damage. It can gain this benefit from resistances only once, and from immunities only twice. And only from resistances or immunities. This cannot reduce the damage below 1. If a creature has taken damage from you since it's last long or short rest, the damage it takes is increased by however much it previously took, ignoring the 100 damage limit.

Secondary effects: if this spell would kill a creature or destroy an object, its remains are destroyed as if by a disintegrate spell. The sound of this spells detonation can be heard clearly everywhere within a mile of the affected area, can be heard vaguely within 5 miles, and anything within 15 miles experiences harmless tremors of the ground. The sky is tinged deep purple within 1000 feet for one minute, and light purple within a mile. The area covered by the detonation, and everything else within 300 feet is heavily obscured by fire for 1d4+1 rounds, and lightly obscured by ash and dust for a further 1d6+2 rounds. Objects larger than 1 cubic foot are only effected on a surface level.

Exhaustive casting: After casting this spell, you suffer 1d4+1 levels of temporary exhaustion. Every 1d4 rounds that pass, that level decreases by 1. If you take more than 10 damage before losing exhaustion, the level instead becomes normal exhaustion. This exhaustion cannot kill the caster, if it would give you level 6 exhaustion, you instead fall unconscious.

Indomitable formation: Any creature who attempts to dispell or counterspell this spell must make a saving throw using their spellcasting modifier or take 8d12 radiant damage as they touch the overstimulated part of the weave that makes up the spell, and there spell is wasted. If this damage kills them, their hands and any spellcasting focus they where using are incinerated.