Salanar the Horseman's dark riders kill captured horses and raise them as Acherus Deathchargers there, while death knight initiates must venture into this realm to defeat the horsemen and take the deathchargers for themselves, a task in which few of the initiates succeed.
Shadowy Tormentors inhabit this dimension, and emerge from it to torment death knights such as Koltira Deathweaver, who implies that it is the Lich King who commands them. He even appears in the World of the Dead, as he calls it, riding a Frost wyrm during  .
Lady Alistra summons Shadow Constructs, Forgotten Servants and a Cenarion Scout from the Shadow Realm to train her Disciples of the Unholy. It's unknown how the Scout came there, but after wondering shortly Lady Alistra kills him too.
The Realm of Shadows may be the same World of Shadows as the one that becomes visible when an is consumed. It may also have something to do with shadow magic and shades. It is apparent that death knights share some sort of connection with it, perhaps even getting their powers from it. If this is the case, then it may also apply to necromancy in its entirety.