User blog comment:Darkcloud1111/The Rules of Death and The Son of Neptune/@comment-3146855-20110909004842/@comment-3146930-20110909005238

Gaea opened the doors to allow the baddest of the bad to return, but that doesnt mean only those picked by Gaea can get through. Also, it may not be the only way as Nico had the chance to bring his sister back by sacrificing a soul for a soul.