User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5112532-20130531134103/@comment-5112532-20130531135653

In the top left corner, you can see a wing and a body.

I'm not sure what it exactly is but I assumed that since they were in Tartarus, that it would be a Fury, but it could be a harpy or maybe something even worse.