Hades is a semi-physical location where the restless dead stay, halfway between life and death.
The realm of Hades seems cold, foggy, and dark. It is not real per se, but somehow captures the spirits of those who died in a way that was shameful or unexpected. They can be of any species, but travel to different circles of Hades when they die.
The circles of Hades are seperate from one another, independently bounded and impossible to travel between. They are vast in size, each holding many millions of spirits.
At the centre of Hades is the portal to true death, which the contented dead travel straight to.
Necromancers can reach into Hades by sending their minds into the place, where they adopt an apparition form and can take the minds of the Dead for their own purposes.
However, while they are in this region, their body slowly dies outside. If they stay too long, they will be trapped forever.