The Realm of Shadows may be the same World of Shadows as the one that becomes visible when a [Elixir of Shadows] is consumed. It may also have something to do with shadow magic and shades. Death knights, necromancers, and undead as a whole share a strong connection with the Realm of Shadows from many hints that can be seen in-game. In the death knight starting zone, death knights are given the task to enter the Realm of Shadows in order to gain their Deathcharger. Later on, death knights are eventually able to learn the ability [Raise Ally] which pulls their ally's spirit back from the Realm of the Dead and forces it back into their body. Upon doing so, a buff is applied to the player stating that: "A touch of the spirit realm still lingers..." This shows that death knights and necromancers both have a connection to the Realm of the Shadows/Dead to perform necromancy which furthers the idea that they actually are drawing their magical power from it.
For the first time since the release of World of Warcraft in 2004, Shadowlands will involve a level reduction ("level squish"), with player characters at level 120 (the level cap in Battle for Azeroth) reduced to level 50, with level 60 being the new level cap (as it had been in the original game). In what Blizzard has called a "New Game+ experience", newly-created characters will have an updated starting experience on an island called "Exile's Reach", which will introduce them to the game and its systems. For players new to World of Warcraft, characters who finish the starting experience on Exile's Reach will proceed to Battle for Azeroth content, while veteran players who create new characters can choose the expansion experience they wish to play through to level 50, at which point they would proceed to the Shadowlands.
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.
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.