Cataclysm (10th level spell)

A crystalline ruby heart, roughly the same size and shape as the material component used, appears in your hands. The heart glows a feint red-orange light and beats every few seconds. It has an AC of 10, 1HP, and immunity to fire, poison, and psychic damage. If the heart is destroyed, it explodes.

As an action, you can detonate the heart from any distance away. During the casting of the spell you can specify a condition that will cause the heart to detonate automatically, such as a creature moving near it or after enough time has passed.

When the heart explodes:

All targets within 2 miles must make a Dex saving throw. A target takes 60d12 fire damage and 40d12 thunder damage on a failed save, or half as much damage on a successful one.

All targets within 2-4 miles must make a Dex saving throw. A target takes 40d12 fire damage and 20d12 thunder damage on a failed save, or half as much damage on a successful one.

All targets within 4-6 miles must make a Dex saving throw. A target takes 20d12 fire damage on a failed save, or half as much damage on a successful one.

All targets withing 8 miles are blinded and deafened until the beginning of your next turn.

The explosion spreads around corners and damages objects and structures. Structures take twice as much thunder damage and flammable objects that aren’t being worn or carried are ignited.

If a structure drops to 0 hit points, it collapses and potentially damages nearby creatures. A creature within half the distance of a structure's height must make a Dexterity saving throw. On a failed save, the creature takes 5d6 bludgeoning damage, is knocked prone, and is buried in the rubble, requiring a DC 20 Strength (Athletics) check as an action to escape. The GM can adjust the DC higher or lower, depending on the nature of the rubble. On a successful save, the creature takes half as much damage and doesn't fall prone or become buried.

The stress of casting this spell weakens you. After enduring that stress, you are unable to cast any spells for 2d4 days. For each of those days that you spend resting and doing nothing more than light activity, your remaining recovery time decreases by 2 days.

(10th level magic exceeds the limits of normal spellcasting and can only be cast by a level 20 spellcaster. As such, it cannot be negated or dispelled by anything short of a wish spell making it immune to magic such as counterspell or dispel magic.)