Uskius' Infinite Calamity Blast

The result of a long and arduous quest, this spell is as perilously close to 10th-level and beyond magic as mortals can come: while even it cannot violate the laws of magic and reality, in it lies the seed of a magic far beyond even its own incredible heights. It operates by fracturing the symmetry of quantum hyper-plasma to generate a destabilized fusion event in an exponential (nearly)unbounded algorithm- in other words, it makes as big an explosion as is magically possible.

Choose a target area in range, and make a ranged spell attack against every creature and object and structure and the terrain in it. On a hit, the spell deals 10d12 Fire damage, which ignores resistances and immunities by creatures and objects and structures and terrain without Legendary or Mythical actions; on a miss, the spell deals full damage, but does not ignore resistances or immunities. This spell spreads through corners and ignores any and all cover and obscurement. This spell requires all of your currently available spell slots.

You can score a critical hit with this spell attack on a roll of 18-20, increased by up to your proficiency for your other spell slots expended- i.e., from 17-20, or from 11-20. On a critical hit, the damage of the previous roll(such as the initial one) is maximized, and roll 5d20, dealing that much more damage; then roll another d20 to determine if that attack is a critical hit(which can also critically hit on a roll of 18-20); if the second attack is a critical hit, the damage of the previous roll is maximized, and roll total of previous attack's d20 for this attack's damage. For example if the initial attack is a critical hit and the second is also, the third attack might use 37d20, the next 413d20, and so on. You can repeat these extra attacks as long as you roll critical hits.

Any structures or creatures or objects or terrain that are not Artifact or Legendary class or that don't have Legendary or Mythical actions and are reduced to 0 Hit Points or fewer by this spell are completely destroyed, only able to return via Wish(specifically to bring them back) or divine intervention; terrain remains damaged until cleared. The explosion of the spell is visible from space and audible all around the planet it is cast on. Deities of magic of the planet's pantheon(or other appropriate entities if such do not exist there) become aware of the spell's casting and that it was you who cast it. The strain of casting the spell is enormous, and you suffer two levels of Exhaustion and fall Unconscious after casting this spell, and you become unable to cast any rituals or magic that requires your spell slots for 1+1d6 days afterwards; you might also gain a shock of white hair afterwards and appear to have aged slightly. You can only cast this spell Proficiency times total during your adventuring career.