User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-79.122.123.16-20130126200003/@comment-79.122.123.16-20130126211418

... and this can mean a different meaning of the prophercy:

"Twins ... the angel's breath, who holds the key to endless death" HOLDS THE KEY to endless death = the sword. Nico is holdong tha sword of Hades (to unlock Hazel).