Demon-dead are members of the Blood who are both living and not living.
When one of the Blood dies there is a possiblity to become demon-dead. If the person has enough residual power and the will to survive their phyiscal body will die while their mind and soul will continue to live on in it. Demon-dead most often retain any wounds inflicted upon death as it is a rule of the High Lord of Hell that the living not interfere with the dead. This rule has been known to be broken on rare occassion.
In order for a dead body to continue to function, the demon dead must consume Yarbarah or Blood wine, or actual blood itself. By siphoning life from the living they are able to maintain their bodies.
It is possible to prevent the Blood from making the transition to demon-dead by dealing the final death to that person. By completely draining their Jewels the possiblity of becoming demon-dead is eliminated.
Hell is the Dark Realm or the Realm of the Dead. Demon-dead are most often found here. They are ruled by the High Lord who for the majority of the series is Saetan SaDiablo . In Twilight's Dawn it is revealed the Daemon Sadi has taken his Father's place as the High Lord.
The Hall exists in all three realms. The one in Hell was created by Saetan in order to offer the demon-dead a place to safely work on unfinished business or simply find peace. In general the living are unwelcome.