The Death Beam of Ultimate Destruction

A flash of crimson light, then a giant ray of pure necrotic power fires from your palm into a 120 foot long ray to a target of your choice. The creature being targeted must make a Constitution saving throw. A creature takes 10d20+100 necrotic damage on a failed save, or half as much damage on a successful one. The caster gains 1 point of exhaustion and 5d8 necrotic damage.