I Have a Book That Can Connect to All Worlds

When Zhang Fan's grandfather died, he gave Zhang Fan a box, saying that it was his family's heirloom and asked Zhang Fan to pass it down from generation to generation. He must never open the box. But I was so curious that I couldn't help but open it. It turned out to be a book. As soon as I opened it, I was inexplicably transported to another world? Which one is real, the doomsday world, the world of cultivating immortals, or the world in which you originally existed? Or is it all fiction? This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.