When everyone was worried about seeing the Catagory King of Diamonds and leaving Joker the winner, why didn't anyone think to unseal the pacifist Tarantula Undead? Sure, Leangle tries to do it after Joker wins and it doesn't work, but nothing was stoping him from doing it before and it would've solved everything!
Tarantula Undead was initially sealed so that Leangle wouldn't be over taken by the Spider Undead. It was only fully successful after Tiger Undead was sealed, so they probably didn't want to take the chance of Spider Undead taking over until Mutsuki was taken out of commission completely at the end of the series.
Okay, I'm only halfway through the series so I don't know if this is answered later, but the Undead's behavior confuses me. Given the premise, shouldn't they be more concerned with fighting each other instead of just running around murdering random humans?