"The Society Against Evil" is a cult where a group of members fight off demons or supernatural beings to save human-kind from the any evil presence or force.
But they had dealt with a greater, powerful, more demonic force that they had fought for over centuries but failed. The demon/presence has been locked in a old house for centuries, the Society Against Evil have failed to get rid of the demon, but before they died, they made a plan. The original owner of the house was another presence that didn't allow Joey Graceffa to sleep but telling him to gather a group of people (Youtubers) to continue the legacy and defeat the demon itself. Edit
Why were they or have to dress in the 1920's with their attitude?
Because during the failure of The Society Against Evil", this took place around 1890-1910. If they have to continue the Society or cult, they must dress up as that time. But was there some sort of prophecy why these Youtubers/people were chosen?
Not exactly, it could've been anyone really. If because there were a prophecy for the Society Against Evil, none of the Youtubers/people would've died. Each episode or part would have to choose 2 members, 1, or more have to complete a challenge or be ready to sacrifice themselves for a clue, an artifact, or anything else (etc.)
One person must or will die per episode. It's what the demon of the house wants to resurrect or create other supernatural beings to kill the other guests (Youtubers/people) or to help him escape or unlock his presence out of the house.
Though, supposedly everyone has to lock the presence or now part of the legacy of The Society Against Evil, there is one person who is also working with the evil supernatural force in the house or how Timothy likes to call it, The Rat.
The Rat is one of the guests and may seem innocent or use the excuse:
"I was chosen for the game, its not me!"
It has been not revealed yet but soon will be during the end of the season, deciphering codes (theorizing), or almost the end of the season.