User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-3146930-20121014230006/@comment-5273810-20121014230617

Well from what I got the Doors of Death are Thanatos and well he can go in and out and no god(except him would go) that is what I got anyway.

As for Echo and the others coming out the Doors of Death I think the Doors of Death are on multiple planes. Remember they have the Depths of Tartarus and Tartarus. I think there is a difference(though am not certain).

And for the finding of the door they are on a quest so...