Board Thread:Questions and Answers/@comment-36592086-20180919032320/@comment-30499831-20180919032648

Aside from "because he's really cool", he could have made the tomb before he was that old. He could have used Earth to tunnel under.

Well.. he could have taken the Realm Crystal from the First Realm. The Oni must have some way to travel between realms. Maybe there are multiple Realm Crystals.