Bobs noble sacrifice

Cast on a willing creature to sacrifice some of their life force to charge a devastating explosion. The amount of total life force sacrificed changes the damage and radius of the explosion. With each 10 hp sacrificed, the range of the explosion is increased by 5ft and 2d8 fire damage. This spell can be charged at the end of the casters turn to further increase the radius by 5ft and 1d8 per turn. This requires concentration. While concentrating on this spell each turn the caster uses their spellcasting ability stat for a DC 14 saving throw to keep concentration. If this save fails the spell detonates immediately.