I just figured everyone should eventually group up somewhere, and figure out a plan for their journey, why they are going on it, all that. Merlin would be the one to tell them 'So yeah, everything's really bad out there and we need help' and it'd be up to everyone as to where they want to go to help, to seal off worlds from the darkness, since that's the main goal of the first game and the reason they will travel from world to world.