This is a mod best reserved for challenges on multiplayer servers or if you want to give yourself a real hardcore experience on your own. Essentially you have five (in game) days to gather as much supplies and materials as you can before the sun burns up the world and evaporates all water on the surface.
This mod will cause you to seriously consider all necessary resources before you’re subjected to an eternity underground. What food will you need? What resources for shelter or tools are needed from the surface? How will you gather materials for farming underground? Definitely consider these things before beginning a new game and then select “Apocalypse” when you create the new world.
Over time grass blocks will start to fade away and water will begin to disappear. So grab what you can.
You will quickly learn how fast a day goes by in the game when time is against you. And once the five days are up you are subject to damage as well. Be sure your spawn point is set underground in case you die.
And once the surface is gone, it’s now up to your wits and what you’ve gathered to continue life underground. If you want to gaze upon the natural light again you’ll need to be under some sort of shade otherwise you’ll burn up quickly. You cannot go exploring in survival mode for very long. Get creative and Good Luck!