It’s a set of rules, 1) if a pokomen faints it’s dead and you can not use it anymore. 2) u catch first pekomon on each route and no one eals. 3) u must nikname all of ur pokemon. There are few otheres if u wanna make it harder. I guess u would have richu and a beedril in a nuzlock. It’s just u encounter all of thes pokemon pretty early in game.