Cluster headache. I had one once that lasted 16 hours. About 12 hours in I was sitting in the closet with a .45 pressed against my head trying to psych myself up enough to blow my brains out so I wouldn't have to feel it anymore.
Luckily these days I have handy little pills that dissolve under my tongue and end it within 15 minutes. It's still extremely painful and horrid, but short at least.