Dying Requiem

Even at the moment before their death, the power of a supreme caster is unparalleled. 

When you are instantly killed, or fail your final death save, you may cast this spell as a reaction. When this spell is cast, select any spell you know lower than 9th level that has a duration longer than instantaneous. You cast the selected spell at 9th level, without need for any of it's prerequisite components, and its duration becomes infinite.  The selected spell cannot be dispelled without the use of Wish, or an effect of similar power.  
Upon this spell being cast, your soul is irreversibly destroyed, its essence used to maintain this final spell.