So I can totally see this series ending up with his 100 disciples chasing after his back while he attempts to avoid them on his journey. His age may be the only reason they don't find him, well in the beginning anyway.
Sooooo...... how does he receive those letters from his apprentices if he has never left the spirit world, or if they can't find him or don't know where he is?
@Rover @Nk9bjP4A He probably gave them a way to contact him so that they could tell him about stuff that they've seen or achieved. After all, wouldn't you want to be able to tell your teacher/master/mentor about the amazing things you've seen or achieved? Plus since his apprentice was constantly doing a ritual in order to know when he came back to the human world, they probably knew where he was but aren't able to travel there themselves. Maybe he gave them magically enchanted envelopes that teleport to him or something.