I'm a Necromancer, Hiding Out and Power-Leveling Like Crazy in the Apocalypse

[Travel between two worlds + all classes + cautious path to immortality + LSP + undead knight + if you’re not horny, hit me] Rich second-generation Fang Yu transmigrates after partying himself to death, landing in a world where everyone can choose a class. He becomes a necromancer. Unfortunately, he has neither connections nor talent, and his prospects are bleak. Then Fang Yu discovers he has a door that seems to let him transmigrate again. And this time, it leads to a post-apocalyptic world crawling with zombies! For a necromancer, it’s paradise! From then on, Fang Yu decides to keep a low profile in the apocalypse, quietly leveling up before astounding everyone. Years later, the legend of the Undying King resounds throughout the otherworld. But... why are all his subordinates undead? At this very moment, all those undead big shots are bowing before that man in his robe.