The Shadowlands has long been referenced in World of Warcraft lore, but their origins and true nature remain a mystery. One thing is for certain; the Shadowlands are inextricably tied to death and decay. It stands opposed to the verdant spiritual realm of the Emerald Dream, which represents a vision of reality untarnished by corruption and mortal intervention.
Explore the Realm of the Dead: for the first time ever in World of Warcraft, players will cross into the Shadowlands and experience the wonders and horrors of the afterlife across five new zones—the gleaming fields of Bastion, the scarred battlefields of Maldraxxus, the eternal twilight of Ardenweald, the opulent keeps of Revendreth, and the horrific Maw.
The spell called "spiritual projection" or "astral projection" brings the caster to the Twisting Nether instead of the Shadowlands; and the Twisting Nether is also referred as "realm of ghosts". However, both planes don't appear to be related, as Outland is floating on the Twisting Nether, but the Shadowlands can be reached in Outland in The Burning Crusade.
Sometimes, the spirits roam the Shadowlands without resting. It can be because their resting place wasn't complete, because of a battle in their resting place or because of the corruption of their resting place. To bring eternal peace to the souls, it is needed to destroy the source of corruption or by making a ceremony with items such as [Tuskarr Ritual Object]. A spirit in the Shadowlands can, with enough power over other souls, tear a hole into the living world once they are strong enough. This is how Jin'do returned to Azeroth.