The flagship show of the CW's Arrowverse, Arrow concluded in 2020 after eight seasons. Arrow had many memorable villains, both created for the show and originating in the comics, featured throughout its run, but each season had its own "big bad" intent on destroying Star City or the Green Arrow that Oliver Queen would take on come the season finale.

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Some of these villains had the power and resources to accomplish their goals and nearly ruin Oliver's life, while others Oliver defeated surprisingly quickly. Each villain was powerful for a different reason, but their strength can't just be measured in abilities. The power they exerted over Oliver's life and the lasting impact they had on him must also be considered.

Emiko Adachi-Queen and the Ninth Circle

Emiko Queen from Arrow

Emiko was the secret paternal half-sister of Oliver Queen. She grew up in the Glades and was trained in martial arts and archery, possessing skills similar enough to Oliver's to briefly become the Green Arrow. She worked alongside Oliver and Team Arrow for a while, helping to capture Ricardo Diaz and free Oliver from prison, before they discovered she was double-crossing them and working with the Ninth Circle.

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She originally wanted revenge against her mother's murderer, but her anger eventually turned to Oliver and his team due to her never being accepted as a member of the Queen family. Despite the Ninth Circle being a seemingly vast criminal organization, they didn't contribute much to the overall storyline. Emiko and the Ninth Circle ultimately fail to destroy Star City and ruin Oliver's legacy, though his family does have to go into hiding to avoid them.

Ricardo Diaz - The Dragon

Ricardo Diaz with his neck tattoo in Arrow

Ricardo Diaz was a ruthless gang leader and drug kingpin who ruled much of Star City's underworld in the sixth and seventh seasons of Arrow. He did not possess any superhuman abilities, but he was a skilled street fighter and martial artist, as well as a cunning criminal lord. Diaz's most notable villainous accomplishment was outing Oliver Queen's identity as the Green Arrow and getting him sent to prison, though his reason for fixating on the Green Arrow was a little unclear.

Overall, he was one of Arrow's weaker villains due to his shaky plot and character building. He was eventually arrested, causing Oliver to be released, and sent to the same prison Oliver had been in before he was murdered by Emiko Adachi.

Adrian Chase - Prometheus

Prometheus on a subway in Arrow

Despite not having any superhuman abilities, Adrian Chase, also known as Prometheus, was one of Oliver's most formidable foes. He was skilled in combat and archery, as well as being very intelligent. His attacks against Oliver were often psychological in nature, wanting to break him more than kill him.

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When it comes to villains accomplishing their goals, Adrian Chase ranks higher on the list than many of the others. Believing that Oliver had ruined his life by killing his father, Adrian set out to take away everything Oliver cared about and he nearly succeeded. However, his obsession with Oliver was ultimately his downfall.

Damien Darhk

Damien Darhk half-hidden in the shadows, smiling in Arrow

In theory, Damien Darhk should be one of the most powerful villains on this list. He was alive as long as Ra's al Ghul, was trained by the League of Assassins, and possessed the ability to control dark magic. He was also incredibly intelligent, but his villainous plan to destroy the whole world except for his "Genesis community" turned out to be far too grand to be reasonable.

In addition, much of his power was tied to the Khushu idol, leaving him vulnerable if it was taken away and vulnerable to other people with magic. Despite those faults, one of the worst things Damien Darhk managed to do was murder the original Laurel Lance. When he ultimately faced down Oliver for the fate of Star City and the world, Team Arrow stopped his nuclear missiles and Oliver killed him after beating him in combat.

Ra's al Ghul - The Demon's Head

Matt Nable as Ra's Al Ghul in Arrow

Ra's al Ghul was the Head of the League of Assassins and, technically, Oliver's former father-in-law. Ra's al Ghul had been alive for hundreds of years, being kept young and strong by the Lazarus pit, and was a deadly martial artist. He trained Malcolm Merlyn and Sara Lance and his daughters were responsible for much of Oliver's training. Ra's seemingly killed Oliver in the season three episode "The Climb" when he stabbed him with a sword and threw him off a cliff. When Oliver survived, Ra's wanted Oliver to become the Head of the League after him according to the League's tradition. Oliver initially refused but Ra's later killed Thea to manipulate him into accepting his offer. Oliver is forced to accept in exchange for Ra's help to bring Thea back from the dead.

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Oliver isolated himself from all of his loved ones in the process of trying to trick Ra's into thinking they were on the same side, causing much of Team Arrow's trust in him to be broken. Ra's was going to destroy Star City when Oliver revealed his betrayal, but their final showdown ended with Oliver killing Ra's, using the same technique Ra's used to stab him, and ironically becoming the Demon's Head anyway.

Malcolm Merlyn - The Dark Archer

Malcolm Merlyn in Arrow

Malcolm Merlyn was one of the most destructive and long-running villains on Arrow. He destroyed half the Glades - killing over five hundred people, was responsible for the death of Oliver's best friend - Merlyn's own son, manipulated Thea Queen into killing Sara Lance, and was the one who originally orchestrated Oliver's shipwreck that caused his father's death. Merlyn was a member of the League of Assassins trained by Ra's al Ghul and eventually became the next Head of the Demon.

Merlyn's experience, intelligence, and convictions often gave him the upper hand over Oliver. However, the real power that Merlyn held over Oliver was more psychological in the long run. Even after Merlyn somewhat redeemed himself before dying in the season five finale, Oliver still regarded him as one of his worst villains. When Oliver and Barry Allen switched lives in the 2018 Elseworlds crossover and were dosed with Scarecrow's fear toxin, the villain Oliver hallucinated that had the biggest impact on Barry's life was, obviously, Eobard Thawne - the Reverse Flash. Oliver's worst villain that Barry sees beating him is Malcolm Merlyn - the Dark Archer.

Slade Wilson - Deathstroke

Slade Wilson in mask and armor, holding gun

Slade Wilson was Oliver's friend and partner on Lian Yu during much of the time Oliver was stranded there. He was also the first person to seriously train Oliver. Even before being dosed with the Mirakuru super strength serum, Slade was an incredibly skilled assassin. The serum caused Slade to lose a lot of his sanity and rationality and made him fixate on Oliver, who he blamed for the death of the woman he loved.

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In retribution for her death and Oliver taking his eye when Oliver tried to kill him, Slade killed Oliver's mother, kidnapped Laurel and Felicity, and unleashed an army of super-soldiers on Star City who wreaked havoc and killed countless people. Once Slade was cured of the Mirakuru, Oliver was able to subdue him, but he chose not to kill him. This allowed Slade's mind to heal and led to him and Oliver eventually becoming allies again.

The Monitor

The Monitor in Crisis on Infinite Earths Part 3

While the Monitor may not be regarded as a traditional "villain," many of his actions throughout season eight of Arrow seem to put him in that category. He took Oliver away from his family, wrecked numerous worlds in order to "test them" before Crisis, destroyed an entire reality - Earth-2 - to make a point, and ultimately got Oliver killed.

That's more than most of Oliver's villains accomplished put together. Not to mention, the Monitor was an immensely powerful cosmic being. Oliver's most heroic moment, sacrificing himself to defeat the Anti-Monitor and rebuild the universe, ultimately comes about because of the Monitor.

NEXT: Arrowverse: 10 Best Villains Ranked