Arrow Season 2.5 Introduces the Church of Blood

While Arrow introduced the character of Brother Blood (and the notion that the role would be passed down through the character's family) in its second season, the notion of the Church of Blood was only just barely hinted at.

The idea, which comes from the comics, is that Blood doesn't just have thugs who do his bidding but is a spiritual leader for an army of acolytes. Since Brother Blood is best known as  Teen Titans villain, a number of those characters (and Azrael, who has ties to Nightwing and the Batman family) have been members of the cult. The cult has ties to Trigon, the demon who is father to the Teen Titans member Raven. Their history is contained with in the Book of Blood.

In Arrow Season 2.5, we find the cult gathered at a dingy, disused building. This may be a nod to their subterranean lair in the comics, where Brother Blood bathes in a pool of literal blood.

They also, as has been hinted by solicitation text, have a Brother Blood at the head of the organization, although whether the man in the "skull mask," as Oliver called it all season long, is the same man is unknown. It seems unlikely that they would bring back Mayor Blood but if they did, it would give Oliver a powerful, major villain who also knows he's the Arrow.

With the district attorney and police chief also killed, the death of the mayor seemed to be setting up the city to be fairly rudderless and leaderless in the second season. The Church, with a sizable following and a charismatic message about "saving" Starling City, could very well be a challenge to established power structures if there isn't a more obvious choice for leadership in place. Then again, that's more or less the same plot they already used last year, so it seems difficult to imagine they'd want to do it again so soon.

The previous volume of Arrow comics took place primarily in their own space, and either in between or during episodes. Not much that took place in them had any impact on the show, but showrunners have said that this time around, material in the comic is fair game to use later on in the show.