User blog comment:Omega natsu2/Chapter 374: Revolution, Predictions./@comment-5598380-20140307101157/@comment-7418318-20140307102723

Something like: "We give you the heart of one of our servants and in exchange, your soul is bound to us, so when you die, you will be ours." ?

So in effect, Hades might have traded his soul for the heart of a Deliora-type Demon.

That actually makes sense, especially with Sayla's comment about selling your soul when you ask something from a Demon and how Franmalth always goes on about the costs of stuff.