The last few episodes of "Arrow" season 3 showed Oliver struggling with the League of Assassins, led by the legendary Ra's al Ghul. Episode 20, "The Fallen," forced Oliver to become a member of the group in order to revive his dead sister, Thea. Episode 21, "Al Sah-Him," is going to reveal the consequences of his actions. 

According to the official synopsis of the episode, the League of Assassins forces Oliver to undergo a radical transformation in his character. The first target they assign him is Nyssa, who is now allying herself with Laurel, Felicity, and Diggle. Now it's Team Arrow fighting against their very leader, Oliver Queen. 

In the promotional trailer, Nyssa mentions that "Oliver Queen is dead" while Felicity ponders if he is under some form of mind control. Whether he is doing it on purpose or if his mind is being manipulated, Oliver puts Team Arrow under a lot of pressure and he might even kill his friend John Diggle. 

During the promotional trailer Oliver is shown standing over a helpless Diggle. Ra's then tells him that the League of Assassins must "eliminate a threat." It wasn't revealed if Oliver pulls the bow and executes his friend, but if he does, it would personify the name the League has given him, "Al Sah-Him," which means "Heir to the Demon." 

Episode 21 may also show the darker side of Thea Queen. According to a report by the IBTimes, it is possible that Thea might turn evil due to psychological effects of the Lazarus Pit. A report from Screen Rant points out that she will reunite with Roy Harper in the episode after "Al Sah-Him," so it is possible she may be saved. 

Fans of the source material would recall that Jason Todd, the second Robin, also turned evil after being revived in the Lazarus Pit and eventually became one of Batman's greatest villains. 

"Arrow" season 3 episode 21 "Al Sah-Him" will air on April 29 on The CW network.