Can a sphenoid retention cyst kill you?

Nope.: It's just a snut bubble in a bony cavity (the sphenoid sinus). No way it will kill you. One way of getting this out of there is snorting saline. (put 1 teaspoon of table salt or dead sea salt in a glass of water, inhale one teaspoon of this concoction up each nostril each morning and evening for a week or two) will most likely fix this (old Ayurveda trick).