The Otherside is one of three planes of existence within the universe. It is where souls of the living are sent once deemed worthy of passing to the Otherside by the endless Death. Those not worthy are sent to the realm of Hell. It's warden is the God of the Universe, A'llye also known as Requiem.
The Otherside is basically the Realm of the Living but with a dimensional barrier that prevents the spirits in the Otherside from interacting with the living and vise versa (with some exceptions such as Shamans). The Otherside is often considered a mirror image to the world of the living, in appearance it appears drained of colour and the Earth seemingly drained of life. Building structures are worn down and animals simply do not exist. A'llye often comments that the Otherside is the world in the future which can be supported by the fact that real time events in the Realm of the Living can alter the landscape in the Otherside.
Despite this, spirits in the Otherside tend to maintain the culture they did in the Otherside, the Kingdom is kept and it's leaders are elected every 4 years. Under the control of A'llye, the Otherside is practically crime free and life there is the closest thing to Heaven the Requiem has.