In each episode there a quest for the hidden scrolls. The quest is to collect the entire different items that are missing to complete the quest.
On each game there’s a different amount that needs to be completed. The counter of the collectibles is located both in the right hand side of the machine:
And on the icon of the game in the lobby:
Upon completion of all the collectibles of the episode the hidden slot unlocks!
The amount of collectibles is tailor made for each player to suit his best experience of the game.