Hades Concept is his Underworld. When he died (Or faded out of existence?) his Underworld (His concept) went with him. we later see him again in Episode G Assassins fending off an attack aganist Zeus while the Main Bronze Saints made Gold Saints fight Zeus. Though Zeus used an attack made to hit the Bronze made Gold Saints, but the secondary effects destroyed the underworld. This is simply to show that Hades did revive for this purpose. Also in the scene where Shura gets rescued. That happend within the Underworld which implies Hades revived..