Realm of Souls

The Realm of Souls is the revered afterlife of the ancient Nehekharans. It was believed that upon death, the deceased's spirit would enter the Realm of Souls -- also known as the Underworld or the Netherworld. Here the Kings of Nehekhara would reside in fatnastic palaces until such a time as a golden paradise worthy of their status was prepared, whereupon they would rule for a million years. It would be filled with all their servants, soldiers and worldly possessions, and so these were necessarily buried beside their monarch upon his death.

However, not all inhabitants in the Realm of Souls would be afforded such luxury, for in the lowest levels, the cursed and the damned would be subject to an eternity of torture for their sins. Only the wretches, the unworthy and the traitors of Nehekharan society were condemned thus, and the idea of those of noble blood could be damned to such a fate was unthinkable.

The Liche Priests of the Mortuary Cult believe that their magical powers originate from the Realm of Souls. With the proper incantations, the Liche Priests are able to summon for the spirits of deceased warriors from this mystical plain and bind them into corporeal bodies to fight for Nehekhara once more.

Source

 * Warhammer Armies: Tomb Kings 8th Edition pg. 14