The Force walk was an ancient Sith technique used to bind restless Force ghosts to the living in order to utilize their strength.
Requiring great strength of will, a Sith could bind the restless ghosts of dead Sith to himself. Once bound, the user could draw from the ghost's power, channeling it in a powerful exertion of Force energy. Binding an unwilling ghost allowed the user to draw more power, while taking a willing one did the opposite. All who have mastered this technique rose to prominence quickly, but then mysteriously faded from glory with no clue as to why. The only Sith Lords who bound more than one ghost were Darth Nox and Lord Ergast himself, whose death also appeared to be a mystery. It appeared that binding too many ghosts can quickly overwhelm the host, inflicting physical and spiritual sickness and lead to violent expressions of Force power beyond the Sith's control. However, if the user could find the cure, which was said to be found on Belsavis and Voss, the user of this ritual could overcome the side effects and can bind as many ghosts as they wished.
300 years after the death of Tulak Hord, the Sith Lord Ergast tried to recreate the ritual that the Dark Lord used to devour the spirits of his enemies at Yn and Chabosh. Instead, he discovered a ritual that he would call Force-walking.