In episode 17, Oliver Queen refused Ra's al Ghul's offer to lead the League of Assassins. The follow-up episode, "Public Enemy," shows that an offer from Ra's cannot be simply denied, especially to his face. 

The synopsis for episode 18 reveals that the League of Assassins has sent numerous copycats to Starling City. The copycats intend to ruin the Arrow's image, forcing him to reconsider their offer, but they end up hurting Ray (recurring guest Brandon Routh) in the process. 

Amidst the chaos Ra's al Ghul kidnaps Captain Lance, and Donna Smoak (Charlotte Ross) comes to the city to visit her daughter Felicity. 

Most of the episode will pit Team Arrow in a race against the police who are hunting down Oliver, assuming that he was responsible for the attack on the mayor. 

The promotional video also shows Captain Lance breaking any hint of truce with Team Arrow. Perhaps Ra's al Ghul only abducts him to make Oliver consider the deal, and this revelation might turn Lance against the good guys. Prior to this episode, Lance and the team have been shakily working together. 

While episode 18 mostly deals with the repercussions of Oliver refusing to lead the League of Assassins, it also leads a little into episode 19. The official synopsis of "Broken Arrow" hints that Oliver and Ray will be forced to work together as they face against a bigger threat. 

According to Screen Rant, it is highly possible that Oliver Queen does take the mantle of Ra's al Ghul. Several set photos reveal actor Stephen Amell in a new outfit, and it is speculated that he could be leading the League of Assassins. 

Screen Rant continues that this decision could be the overhaul to "Arrow" that Amell previously hinted at. Perhaps the season finale may turn Oliver into the main villain in season 4, with the seeds of that event being planted firmly in episodes 18 to 20.