I was shooting a haunted house in complete darkness and needed the rare time to use the AF assist lamp but couldn't get it to turn on. I have a nikon D800 and D300. D850 has some infrared thing not sure what that does.
It's under the menu at A8, yes i looked at manual also. What are chances both of them blew out? seems unlikely when i never used it.

Doesn't the AF lamp work only with single point auto focus with the center point selected?  Is that what you are running?  Check that.  Running anything between your body and your speed light?  Check that it is something suitable, such as SC-29 cord and check whether it is switched to be the assist system.  If using SB-900 or better, I would use the speedlight as the assist system.  The range for the actual lamp on the camera body is only something like 1.5 - 10 feet for most lenses but am not sure if this would affect whether or not the lamp would turn on at all.