The dungeon was unknowingly corrupted by some Obsidian Virus and now you must explore the depths and defeat skeletron prime to proceed to the fight with the moon lord deep underground and once you enter the dungeon there is no escape (unless you miss the checkpoint's and die but I don't recommend to really explore cause there is nothing interesting) you can also use it as a survival that starts in hard mode after collecting the hard mode ore's but with an odd dungeon that contains life crystal's and life fruits that have moon lord summons. It was meant to be something with islands but I scrapped it so the world is Island Survival (which is my next project)
