Vishra's Molecular Fission

Developed by the legendary Realmwalker Mage Vishra the Bloodless, in her arsenal of personalized arcane magic. The most brutal, most harsh, and vile of all spells she created, even she tried to seal it away forever. Very few would dare attempt it, for the sheer cost it has on the caster. The spell causes the caster to use their own molecules as fuel for an arcane fission reaction, which creates a force of energy strong enough to level kingdoms. To use it would mean instant death for the caster and everything else around. After being cast, the caster must spend at least 18 seconds focusing on the complex spell. It is not concentration, but the spell is disrupted if the caster falls unconscious before the spell is finished casting. 

When the spell finishes, the caster can release the energy, or hold it for up to 3 more rounds before it releases automatically. There is no dispelling it. Once it is cast, it is cast. There is no going back. You will die. 

When the spell is released, every creature and structure within a 200-foot radius of the caster (INCLUDING THE CASTER) takes 10d4 * 100 (1000d4) force damage that cannot be reduced in any way. If they are within 100-feet, the attack is considered a critical. If the target drops below 0 HP, they disintegrate on the spot (also applies to the caster).

Created by the Bloodless: Reroll any 1s on damage dice for any spell from the learned mind of Vishra the Bloodless. 

"Of all my works, this is the most vile. To you who finds this, I know your quest. I know your pursuit of power, of the ability to hurt your enemies, and to protect the ones you love. This spell cannot be allowed to exist. But I'm too weak to do what should be done. Destroy this spell, traveler. This is not worth it. Believe me, I know. Even death is better than the fate that lies the poor soul who casts it. They will never reach an afterlife. Existence. You'll never know that again. Don't **** this up."

-Vishra, the Bloodless