Arrow has retconned the Anti-Monitor as the cosmic being who founded the League of Assassins. The League of Assassins have been one of the most dangerous recurring threats in Arrow, an ancient guild who have killed thousands over the centuries.
Arrow season 8, episode 3 saw Oliver Queen investigate the ancient history of the League of Assassins. He learned that they were created back in 1013AD, when a cosmic being approached a man named Al-Fatih. This mysterious entity charged the League of Assassins with maintaining a balance between good and evil across the globe. If they failed in their mission, then he warned that he would “bring forth the end of times, the annihilation of all things.”
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The Book of Al-Fatih didn't name the being who appeared to the League of Assassins' founder, but there was a sketch. Oliver thought he recognized this as the Monitor, and concluded that he'd been misled. He'd believed he was working with the Monitor in order to save the Multiverse, but now faced the possibility that the Monitor is the one who's actually destroying entire realities. There was just one problem, though; on closer examination, the face in the sketch isn't the Monitor at all. It's the Anti-Monitor, another entity who none of the heroes know exists. The Monitor and the Anti-Monitor are visually similar, so it's easy to understand why Oliver wouldn't realize this is a different being.
This is an important retcon, because it reveals that the Anti-Monitor has been personally intervening in the history of Earth-1. Indeed, he established the League of Assassins a thousand years ago, and they've presumably been serving his purposes for a millennium. What's more, the League's code – their mindless obedience to whoever bears the Sword of Al-Fatih – will have been shaped by the Anti-Monitor as well. That sword is currently wielded by Oliver Queen's sister, Thea, but the Anti-Monitor probably has his own plans for it. The Anti-Monitor possesses the power to scoop people from any point in the timeline. He could summon any member of the League of Assassins from history, and they will be sworn to serve whoever holds that blade.
This retcon subtly rewrites the history of Arrow, revealing that Oliver Queen has been fighting on the front lines of a Multiversal Crisis for as long as he's been a hero. Almost all the threats that have faced Star City were rooted in the League of Assassins, or else in the actions of rogue members such as Malcolm Merlyn. Though Oliver doesn't know it, he's been battling against the Anti-Monitor's forces all this time.
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