The End of the World: I Build a Tens-Billion Fortress

[The end of the world + no heroine + relaxed and funny + system + cultivating immortals in a different world] Five hundred years ago: I may not be able to go back alive! Five hundred years later: You may not be able to go back today! Because my Immortal Killing Sword Formation will shine on you and your sect's useless sect-protecting formation, may the shadow of death always be by your side!