This challenge series is going off the idea that in ancient times people used the monsters in their soul to do battle, similar to how people use aura to fight. So for these ideas people are either going to have one monster from their soul or their soul makes a deck for them to use. the second idea would be good in the case of people having duels or "spars". For actual fights they would summon a monster from their deck to fight the grim but for spars it would follow the traditional duels.