User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10137020-20130620135443

I was reading the Son of Neptune and I realised that Thanatos may help Percy and Annabeth close the doors of death since he always uses them to travel between the mortal world and Underworld. So he must be close by when Percy and annabeth are in their journey to find the doors of death.