Let me explain something since you seemed confused. If your Pokemon loses a battle, against anyone that works for Slade, who are the only other humans in the dark world, no Team Galatic copies, they will lose their Legendary Soul and will never get that power back. They are the only people there and they don't have Pokemon like Slugma and Cubone or Nuzleaf, they have high level fully evolved Pokemon. They work for Slade, they do not want to capture him, and also not other Pokemon live in that world other than Pokemon with a Legendary soul listed above.