The show's big season five villain turned out to be someone close to Oliver (Stephen Amell) and his team, so executive producer Wendy Mericle breaks down what this means moving forward.

[This story contains spoilers from Wednesday's Arrow, "Fighting Fire With Fire."]

Arrow's big bad of the season has finally been unmasked, but Oliver (Stephen Amell) still has no idea that his biggest enemy is masquerading as his ally.

The CW's superhero drama unmasked serial killer Prometheus and it's none other than District Attorney Adrian Chase (Josh Segarra), the man who befriended Oliver through their shared work in the Mayor's Office. What's more, he helped get Diggle (David Ramsey) out of jail and always stood up for what's right — at least on the outside. In reality, he was hiding his intentions and identity: as the son of a man Oliver killed in season one when he was operating as the Hood.

In the DC Comics lore, Adrian Chase is actually Vigilante, an anti-hero who has been stirring up trouble in Star City all season long. The reveal is a shocker as Arrow showrunners opted to subvert fanboy expectations and make Adrian's secret identity Prometheus instead.

"One of the reasons we did do it was because everybody would be thinking, 'Of course he's going to be Vigilante,'" executive producer Wendy Mericle told reporters, including THR, during a recent conference call. "We thought it would be a really fun twist to do what we've always done on the show, which is to take the comic book mythology and turn it on its head and see what kind of story we can mine from a surprise like that. It was also something different for this season. We wanted to change up how we introduced the big bad and change up when we did it."

Unlike last season's grave mystery, where the writers didn't know who they'd be killing off after introducing that storyline in the premiere, the showrunners planned for Adrian to be Prometheus from the beginning.

"We always knew that Adrian was our big bad," Mericle said. "We always knew in our very first discussions, which were started about a year ago. ... We wanted to go back to season one and explore the roots of Oliver's mission and the consequences of everything he's done."

While viewers know Adrian's secret, Oliver and Team Arrow are in the dark that one of their closest allies has been working against them. Adrian has known Oliver's secret identity, and after turning Team Arrow's Artemis (Evelyn Shaw) to the dark side, he also knows all their vulnerabilities. He has all the power, and will enjoy holding that over Oliver and his team moving forward as they try and scramble to figure out who this mysterious hooded killer is. And, at least for the time being, will always have the upper hand.

"Ultimately, the bond between them is really strong, which we hope will create a much bigger impact on Oliver when he finds out down the road who Adrian really is and that this is the guy who's been really messing with him all season," Mericle said.

But is Adrian a bad guy? Oliver is technically a serial killer, and his actions in season one actually created this new serial killer since he killed Adrian's father and set Adrian's plan in motion. So what right does Oliver have in judging Prometheus?

That will be explored this season, Mericle said, noting that producers will try to humanize Adrian to give him some empathy.

"We've always striven to have the villain be the hero of their own story and to get underneath the reasons why they are what they are and do what they do," Mericle said. "Adrian Chase is no exception. What's interesting for us is how close those parallels are with Oliver's own story. We wanted to build a villain that's a dark mirror for Oliver, and honoring Oliver's past on the show, especially the first season. Seeing what it was like going through the loss. They've both suffered losses and they've both answered those losses in very different ways."

What sets Prometheus apart from the other big bads Oliver has defeated over the years is the fact that he's not trying to kill Oliver. He wants to destroy everything Oliver loves in a sick game of psychological torture.

"We really wanted to build him up as someone who is extremely good at being 10 steps ahead of Oliver, really being psychological in the way that he manipulates him," Mericle said. "One of the ways he does that is by going after the people that Oliver cares about. We’ve seen villains do that in the past, but this year Adrian does it in a way that’s a little bit more specific. His goal is very clear: he really has no drive to kill Oliver. It’s purely based on torture. It’s all part and parcel of this drive to break Oliver down psychologically as opposed to physically, which we have seen Slade [Manu Bennett] and Merlyn [John Barrowman] and everybody else do, take him on hand-to-hand. This is much more of a mental game."

Did you see Arrow's big Prometheus twist coming, or did it take you by surprise? Come back to THR's weekly DC TV Watch column on Friday morning for Segarra's thoughts on the big reveal.