User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-74.134.155.110-20121109054511

Here's something to think on: The Sword of Hades short story in the Demigod Files. The sword was made with one of the Keys of Hades, which can "lock or unlock death".

Keys.

Doors.

"Twins snuff out the angel's breath,

Who holds the key to endless death."

I'd also like to point out that Nico stayed behind at the end of the short story. It's just a small thing that I noticed, but I think it's interesting.