The Great Expanse

You unleash a catastrophic explosion, dealing massive damage to all creatures in a radius around you. In the process, you sacrifice 70% of your total health, regardless of your current hit points, to fuel this devastating spell. Shrapnel of "who knows what" flies everywhere as you detonate causing extra piercing damage. Any creature with the radius of your explosion must make a Constitution saving throw to endure the shrapnel and explosion as there is no chance for them to dodge this attack.