User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-58.106.236.75-20121204101115/@comment-3146930-20121222134518

In order for the Doors of Death to be closed, they must be closed from both sides, otherwise they won't close at all. One set of doors is in the House of Hades while the other side is in Tartarus currently. So someone has to stay behind and close them. Most people think that it has to be one of the seven, but you never know. Maybe they will meet someone in Tartarus that will do it for them. Maybe a Titan trying to redeem themselves.