The Underworld Garden is the private realm of Vadriel. No being but Vadriel and Omega can access it, unless Vadriel either summons them or the single portal is used. Vadriel resides in it currently, gaining power while Omega is attempting to revive him. The Garden is fueled by the three dark glyphs, Acerbatus, Morbus, and Umbra, and without them, the garden will fall apart and revert to its true form, which is fueled by Dominus.
The Garden is beautiful in appearance, and there are several rivers and/or lakes. However, it is deadly, as most of the plants are poisonous, and the rivers are filled with skeletal swordfish, and the deadly Reptilicus demons.
Even this appearance is false, for without the glyphs, the floor is battered and damaged, there are holes in the walls, the plants (or what is left) are demented and withered, and the sky is red. There are only a few full rooms in the garden's true form, including Omega's room, the Golden Room (links to Vadriel and Omega's rooms) and Vadriel's room (Vadriel's room is false as well).
In an upcoming fan-game, the Underworld Garden is a secret area. The theme for the first part will be "Garibaldi Courtyard" and the theme for the damaged part will be "Aiolon Cave Temple". The theme for the Golden Room will be Arduous Journey. There will be three major encounters with a mysterious character who holds one of the three glyphs (in the order of Umbra, Acerbatus, and Morbus), and after the last encounter, the area will become damaged. After the damaged part is passed, the Golden Room can be entered. The room features two paintings, a blue dragon and a golden skull (the second of which is locked). The Blue Dragon leads to Omega's room. After Omega's defeat, the Golden Skull is unlocked, which leads to a short mini-area, which finally leads to Vadriel's room.