This mod adds a gamerule ("Darkness Kills Players") that determines whether players will die in a light level under 6. It has to be enabled first and isn't turned on on default.
This was inspired by Possible History on Youtube, by the video "What If Darkness Literally Kills Humans?"
where he theorizes about how history, humanity and civilization would've developed if darkness literally killed humans by itself.
Don't take this mod too seriously, I made it in like 15 minutes but let me know about any bugs you find.