All spoilers point to an ominous ending to the current season of "Arrow." Actor Stephen Amell talked about his character Oliver Queen joining the dark side, and executive producer Marc Guggenheim described the bad things that are still going to happen in the remaining episodes. 

The official synopsis of episode 21 titled "Ah Sah-Him" goes:

Oliver goes through a rigorous transformation. The League of Assassins sets their sites on Nyssa, who is back in Starling City training Laurel. When Nyssa realizes they are coming, she arms up to face them alone, but Laurel asks Felicity and Diggle to help protect her new friend."

The synopsis already confirms the evil things coming, with the mention of the "rigorous transformation" and of Felicity and Diggle, who looked very much in danger in the sneak peek. The most scary thing is, it is Oliver Queen standing in front of the kneeling Diggle, and it looks like he is going to kill him under Ra's Al Ghul's orders. 

Guggenheim talked to TVLine and confirmed that Oliver has to "do some stuff" in order to stay in The League of Assassins. As the executive producer said, "Once you're in, you're in," a thought that is blowing every fan's mind right now. How can Oliver be the hero Arrow if he's gone evil? 

Even Amell (Oliver) tweeted his confirmation to where his character is going. On April 16, he tweeted "Evil Oliver. Coming Soon." 

Guggenheim also shared a couple of new details to look forward to in the last episodes of season 3. He mentioned that "you'll have to wait and see, on May 6" if there will be an Oliver and Felicity wedding happening. As for the season finale, the executive producer promised that it will be something different. Not only will this put an end to the previous seasons, but it will offer "a very strong indication of who the Big Bad for Season 4 will be."

"Arrow" season 3 episode 21 airs on Wednesday, April 29, at 8 p.m. on The CW.