The Church of Blood was a secret following founded by Roger Trigon, later led by Sebastian Blood. In reality, Blood acted on the orders from Slade Wilson, who used the cult as means to find and recruit people able to withstand the effects of Mirakuru, whom Slade would use for his army. Blood eventually abandoned Slade Wilson after seeing the danger Slade's Mirakuru soldiers posed to the city, only to be killed by Isabel Rochev for his betrayal. After Slade was imprisoned by A.R.G.U.S. and Isabel was killed, leadership of the Church was taken over by Clinton Hogue, who claimed the mantle of Brother Blood.
15 years before the plot to take control of Starling City by Deathstroke, the Church was led by Father Roger Trigon, head of Zandia Orphanage. Trigon appeared as an affable religious leader, which allowed him to recruit orphans and the city's poor into his mystical organization. Appearing in his alter-ego as the Devil, along with the Acolyte, Cyrus Gold, Trigon organized the murder of Sebastian Blood's abusive father, cementing Blood as the member of his church. According to Trigon, the Church's goal is the greater good for all the poor and the weakest of the city, no matter of the bloody cost.
Sometime after The Undertaking, Sebastian Blood, with his supply of Mirakuru coming from Slade Wilson, reformed the Church in Starling City, and claimed to be the leader of the Glades as its alderman. He then recruited people into his cause and decided to use an army "of the strong", basically consisting of those who survived his test.
On the outside, Sebastian Blood as an alderman, he was a well-liked, charismatic person who knew how to win a crowd within the Glades, and had the support of those who had been devastated by the Undertaking. Sebastian wanted control of Starling City to achieve the dreams of the Church of Blood, and had an apparent vendetta against the Arrow, as he was disappointed by the news that Cecil Adams was unable to kill him after being resupplied with his operations in Starling City by Blood.
The real mastermind behind the activities of the Church in 2013 was Slade Wilson, who was using it as an instrument to help him exact his vengeance on Oliver Queen for Shado's death. After providing Brother Blood with the Mirakuru, used in the practices of cultists, Slade gave Sebastian orders not to engage the Arrow, while preparing soldiers able to survive the injection of Mirakuru. Slade ultimately used the soldiers for his own loyal army that would destroy Starling City, leaving Brother Blood with nothing and causing his rebellion.
Sometime after the Undertaking, Sebastian Blood supplied Cecil Adams with what he needed to restart his Vertigo operations, but by the time Oliver Queen returned to Starling City after being on Lian Yu following the events of the Undertaking, Sebastian has become a well liked person and the people of the Glades seeing him as a leader with signs saying "Blood for Mayor". At some point, he hired the Hoods to assassinate the mayor of Starling and clear the way for his own mayoral candidacy. 
Sebastian has criticized Oliver Queen in the media after he was unable to attend an event he put together to make amends with Sebastian, who didn't have a very good opinion of him and the Queen Family. 
Sebastian also sent Cecil Adams to kill The Arrow only to fail and be killed himself, one of his agents reported that there was progress, after dressing up in his costume, Sebastian and Daily approached one of the surviving members Cyrus Gold asking how he felt, when he said he felt stronger, Sebastian told him he was ready to begin. 
Cyrus Gold, now infused with Mirakuru, began immediate assaults on Queen Consolidated's applied sciences division and a blood bank for equipment to mass-produce the Mirakuru. On the third stop which was an A.R.G.U.S. bunker, he attacked Oliver Queen using his strength which caused Oliver to slam into a shelf before being drugged and left for dead. 
Meanwhile, Officer Daily followed Sin and Roy Harper who were looking into the death of one of Sin's friends who was a member of the Church, Max Stanton, that ended up dead from the effects of Mirakuru. Cyrus Gold knocked out Roy after killing Lucas Hilton and injuring Quentin Lance during a raid to capture him, after being found out by Oliver Queen. 
Sebastian injected Roy with the Mirakuru as Oliver arrived to save him. Oliver managed to finally slay Cyrus with an arrow to the centrifuge that was producing the Miracle. Roy was brought back to life by Oliver after performing CPR.
Later on, Sebastian announced his decision to run for Mayor of Starling City on television while he went to meet his shadowy benefactor, who told him that he will provide him with another sample of his blood to produce more Mirakuru. The mysterious benefactor is revealed to be none other than Slade Wilson, who had been presumed dead. Slade orders Sebastian that to not engage the Arrow directly, or Slade will find another to wear his cult mask. When questioned, Slade tells Sebastian that he will take away everyone and everything he cares about and when that is done, he will personally drive an arrow into Oliver's eye.
Sometime later Sebastian murdered his mother Maya Resik after she told Laurel Lance the truth about Sebastian, that he murdered his father and placed her in a mental ward. Afterwards, Slade ordered Blood to handle Laurel, which resulted in Daily arresting her for possession of controlled substances. 
Upon her release, she is captured by Sebastian and sent to an unknown location where she was going to be killed by a man wearing Blood's mask. The Arrow and Laurel defeat the masked man, who is revealed to be Officer Daily. Blood then returns to Slade who tells Blood that he failed once again, killing Blood's two bodyguards and pointing the tip of his sword at Sebastian's throat, warning him that if he failed one more time, then he would be the next to die.
Slade saw the news report of Moira Queen's announcement of her entry into the race for mayor; he reminded Sebastian not to underestimate her. Sebastian met with her and threatened her to withdraw from the race and give up on winning, wondering which Moira Queen would be the face of the campaign: the Moira who was the victim of Malcolm Merlyn, or the one who could lead the city away from fear. 
Slade told Sebastian that he will handle Moira when he questions what they will be doing moving forward.
Later on, Isabel Rochev used Thea's kidnapping to trick a distracted Oliver into handing control of Queen Consolidated over to her, and had the board of the company oust Oliver and appoint her as permanent CEO. She briefly battled Oliver, revealing that she has been working with Slade Wilson all along and that she only bought Queen Consolidated's stock to lure Oliver back to Starling from Lian Yu. It is also revealed that the cultists that the Church of Blood experimented on were needed by Slade for the creation of his Mirakuru-augmented army. 
Meanwhile, Deathstroke hijacked a prison bus headed for Iron Heights Prison. He killed the guards and freed the prisoners, recruiting them into his army. Later on, he met with Sebastian Blood and Isabel at Queen Consolidated, where Isabel confirmed that Queen Consolidated's applied sciences division is now dedicated to mass-producing Mirakuru, which they would use to create an army of superpowered soldiers to conquer Starling City.
Deathstroke had to look for an alternative way to infuse his army of prisoners with Mirakuru after Team Arrow blowed up Queen Consolidated's applied sciences division. He captured Roy, infiltrated the Arrowcave, attacked Team Arrow, and stole the Clock King's skeleton key. With the key, he broke into S.T.A.R. Labs and stole a bio-transfuser which he used to transfuse the Mirakuru from Roy's blood into his army. Eventually, the Arrow tracked down the bio-transfuser with Felicity's help, and confronted Slade and Isabel. Oliver managed to shut the machine down and escaped with Roy. During the battle, Isabel was shot through the chest by Diggle. Later, Slade was seen having used his own blood to complete the Mirakuru transfusion into the prisoners and Isabel, who was revived. 
Set on completing his revenge, Slade caused a limo carrying Oliver, Moira, and Thea to crash. He then captured all three of them and then re-enacted the occasion of Shado's death, this time forcing Oliver to choose whether to kill Moira or Thea instead of Shado or Sara. Eventually, Moira offered to sacrifice herself to spare Oliver and Thea. Slade commended her courage, before killing her with his sword. 
His plans fruitful, Sebastian Blood is sworn in as mayor, having won the elections after Moira's death. Isabel visited the reception to Moira's funeral to taunt Diggle and Felicity, telling them that they will be the next to die. Isabel managed to have Verdant taken from Thea. Laurel planted a bug in Sebastian's office which allowed the police to hack his computer, discovering a press release offering his condolences to the Queen family, dated the day before Moira's death - incontrovertible proof that Sebastian knew that Slade was going to kill one of the Queen family which Laurel told Oliver about, making Oliver realize that he had been deceived this whole time. 
Armed with this information, Oliver confronted Sebastian and revealed that he knows Sebastian is working with Slade Wilson, as well as his secret identity as the Arrow. Sebastian claimed that after Slade's army is done with Starling, the people will flock to him for leadership, and he can rebuild the city according to his vision, but Oliver tried to convince him that Slade would not deliver on his promise and that he turned the Church into a pawn, though Sebastian refuses to listen.
Later Brother Blood rallied Slade's army of Mirakuru-enhanced soldiers, preparing to lead them in their assault on Starling City. One soldier was sent to the police station, where he killed Kelton and battled Quentin Lance, while another was sent to attack the train station where Thea happened to be. Team Arrow attempted to destroy Slade's army's hideout with explosives and bury the Mirakuru-enhanced prisoners and Blood inside, but before Diggle could trigger the explosion, he was attacked by Isabel, as Ravager.
Slade's army continued tearing apart the city. Felicity rescued Diggle by running Ravager over with a van and driving away with him; Ravager survives thanks to the Mirakuru. Slade overheard Oliver talking to a S.T.A.R. Labs courier who is carrying the Mirakuru cure, and sent his soldiers to retrieve it. As Oliver, Diggle and Felicity drive to the courier they are attacked by Mirakuru soldiers who topple the car and Felicity is knocked unconscious, but Oliver uses an explosive arrow to knocked out the attackers. However the Mirakuru soldiers arrive to the courier, kill him and steal the cure before Team Arrow can get there. Laurel Lance is almost cornered by a soldier but Sara Lance/The Canary. Meanwhile, Quentin Lance stops the Mirakuru soldier in the police station by blowing him up with grenades, and Malcolm Merlyn/Dark Archer kills a Mirakuru soldier at the train station to save Thea. Malcolm then faces another Mirakuru soldier, or possibly the same one, trying to kill Thea and manages to kill him. 
At the mayor's office, Sebastian Blood is commanding the city's response to Slade's attack, convincing D.A. Spencer that he has things under control. However, one of Slade's soldiers bursts into the mayor's office and kills Spencer, despite Sebastian ordering him to stop. A furious Sebastian confronts Slade, saying that he wanted only to cause enough damage so that he could rebuild Starling City according to his vision, not destroy it completely. Slade is coldly indifferent and insists that the only thing that matters to him is hurting Oliver Queen, and that the total destruction of Starling is part of that aim.
Sebastian responds by stealing the Mirakuru cure and delivering it to Team Arrow. He justifies his actions to Oliver by saying that he has always wanted what's best for Starling City and vowing to redeem himself by rebuilding it after Slade is stopped. An incredulous Oliver asks him if he really thinks the city will let him continue as mayor after what he's done; Sebastian replies that no one but Team Arrow knows of his involvement and threatens to reveal the Arrow's secret identity should Oliver ever tell the world of Sebastian's actions as Brother Blood. Isabel then arrives at the mayor's office, asking Sebastian where the cure is. When he tells her he has already given it to Oliver, she stabs him to death. Slade's army attack causes Amanda Waller to send A.R.G.U.S. troops to secure the city exits and prepare an airstrike from a predator drone.
As the attack continued the Mirakuru soldiers attack the church tower where Team Arrow is, after successfully testing the cure on Roy, and escape by sliding down a cable as Lyla Michaels arrives to blow up the tower. The Arrow and the Canary using cure arrows invade Queen Consolidated where Deathstroke, Ravager and a dozen Mirakuru soldiers are waiting. However they are surprised by the arrival of the League of Assassins lead by Nyssa al Ghul, who kills Isabel, while everyone else is cured though Deathstroke manages to escape. On the news Bethany Snow reports Sebastian's death. 
The Mirakuru soldiers aware of the impending airstrike were ordered to escape through the tunnels while one injured Quentin and kidnapped Laurel from the S.C.P.D. Arrow, Canary, Roy, Nyssa and the League attacked the army in the tunnels and cured most of them. As one was about to kill Sara, Nyssa cured him but as another almost killed Nyssa but Quentin arrived in time to cure him and reluctantly saved Nyssa. Meanwhile, Deathstroke kidnapped Felicity believing her to be Oliver's true love, as part of a deception between the two.
Slade forced the Arrow to arrive alone where he and a soldier had Felicity and Laurel but Felicity cured Slade while the Canary cured the soldier and got Felicity and Laurel away as Deathstroke fought the Arrow. Eventually after a prolonged fight Oliver was able to restrain Slade to a pillar and ordered Waller to stop the airstrike, before incarcerating Slade in an A.R.G.U.S. super-max on Lian Yu itself.
Despite the destruction of Slade Wilson's army, the Church itself survived the death of both its leader and its secret mastermind, as Clinton Hogue claimed the mantle of Brother Blood for himself, and restored the cult, before eventually being stopped by Team Arrow.
- Langford/Brother Langford
- Abel (second-in-command under Clinton Hogue; deceased)
- Sebastian Blood/Brother Blood (third-in-command, official leader; deceased)
- Daily/Brother Daily (spy within Starling City Police Department; deceased)
- Cyrus Gold/The Acolyte (deceased)
- Caleb Green (deceased)
- Roy Harper (unwilling test subject)
- Clinton Hogue/Brother Blood (former leader; deceased)
- Xavier Reed/The Mayor (unwilling test subject; deceased)
- Isabel Rochev/Ravager (spy within Queen Consolidated; second-in-command of the Church of Blood and in the Mirakuru army; deceased)
- Max Stanton (unwilling test subject; deceased)
- Hoshaw Tolibao/Brother Hoshaw (incarcerated)
- Roger Trigon/Father Trigon (founder; deceased)
- Slade Wilson/Deathstroke (de-facto leader and provider of Mirakuru)
- Mirakuru army (soldiers; most incarcerated, some deceased)
- I. Vasak (scientist; deceased)
- Team Arrow
- League of Assassins (disbanded)
- Moira Queen (deceased)
- Amanda Waller (deceased)
Behind the scenes
- In the DC comics, the Church of Blood, also led by Sebastian Blood, worships the demon Trigon, the father of Teen Titan Raven.