Board Thread:General Discussion/@comment-6192846-20150326001739/@comment-6986530-20150416030130

He also said outright that Grim the End was the greatest of his creations, confirming not only that he created the Golden Key by implying he made the other vessels as well. Seth also stated his specialty was the transference of souls, so I don't see it as too unlikely that he put the other demons inside once he found them.