After using Lilith to create a army of Artificial Devils, those Devils started to reside in the Realm of the Dead and served as Hades' troops in his war against the Tree Factions and Team DxD. It's also where Vali's great grandmother Lilith is being held captive by Hades.

There is a carving on the wall of the trinity Gods of Olympus – Zeus, Poseidon and Hades and it is carved to stand out. Ornaments such as gold are used for the interior and it's so gorgeous that it doesn’t suit being in the Underworld. There is a part in it that is something that looks like a ritual area. It is the place where the Grim Reapers of this World live and where the castle of Hades is located. It also has an ancient Greek shrine as it appears into the depths. It isn't as big as the Underworld and as it's the World of the dead, it is a wasteland that no living creature can live in.

The realm of the dead is of the Olympus World the God from the Greek-faction Hades is in control of it. Located in the lower strata of the Underworld and the place where the souls of the dead are sorted and is also close to Cocytus.

Highschool of the dead wiki