"Arrow" Season 4 will see Oliver Queen (Stephen Amell) reunite with his former sidekick Roy Harper (Colton Haynes). The CW

With the battle for Star City in full swing, the team on the CW’s “Arrow” will undergo a small reunion in episode 12 of Season 4 when two incredibly valuable members of the team return. However, with the heroes coming back into the fold, a new villain from DC Comics will be there to put a damper on the welcome home party.

After leaving the show in the middle of Season 3, Roy Harper (Colton Haynes) will make his triumphant, and highly anticipated, return to “Arrow” as the red-hood-clad sidekick Arsenal. Harper forced himself into exile after taking the fall for Oliver Queen (Stephen Amell) as the hooded vigilante of the city. As a result, it’s unclear exactly what brings the character back into the fold, but odds are good that it spells trouble for Team Arrow.

"Arrow" Season 4 characters Roy Harper (Colton Haynes) John Diggle (David Ramsey) and Laurel Lance (Katie Cassidy) will team up in episode 12 to take down a new villain. the CW

According to TV Guide, the episode will see the introduction of an exciting new villain, the Calculator (Tom Amandes). The villain is described as being a bit of a super-genius that’s capable of throwing Team Arrow through a loop, particularly its shakiest member right now, Felicity (Emily Bett Rickards). According to Entertainment Weekly, many believe that the Calculator may turn out to be the newly code named Overwatch’s long lost father. However, with a highly intelligent villain coming to Star City, it’s possible that the return of Roy at the same time isn’t just a coincidence.

Meanwhile, the episode also marks the return of Nyssa al Ghul (Katrina Law) and teases a bold, game-changing move on her part. The last time she was on the show, she swore vengeance against Malcolm Merlyn (John Barrowman) after he allowed the love of her life, Sara Lance (Caity Lotz), to be resurrected in the Lazarus Pit. Nyssa, filled with rage at her master’s decision, used some mystical means she’d been hiding to close the Lazarus Pit forever, much to the dismay of Merlyn. The villain took her into the custody of the League of Assassins and viewers haven’t seen her since.

Fans hoping to find out what brings both Nyssa and Roy back into the fold will just have to tune in to “Arrow” Season 4, episode 12, titled “Unchained," to find out. The show airs at 8 p.m. EST on Wednesday.