Deathstroke played a big role in the earlier seasons of "Arrow." He has not been seen on the show since his imprisonment many seasons back, but something has resurfaced online that hints at the villain's possible return to the action-packed series.

Starting out as Slade Wilson (Manu Bennett), the masked villain named "Deathstroke" used to be a friend and an ally to Oliver Queen (Stephen Amell) during the time he got stuck on an island called "Lian Yu." Slade even taught the shipwrecked billionaire how to fight in combat, which came in handy when Oliver became the Green Arrow. However, later on, Slade was injected with an experimental World War II serum called "Mirakuru," which gave him super strength but then warped him into a hostile and aggressive warrior at the same time. Oliver, on the other hand, was placed in a sticky situation in which he had to choose between his close friend and former lover Sara Lance (Caity Lotz) and his new lover Shado (Celina Jade), whom he met on the island. It just so happened that Slade had feelings for Shado as well, and when he discovered that Oliver chose to save Sara's life instead of Shado's, he swore to do whatever it takes to kill Oliver.

In order to defend himself in combat, Oliver "killed" his friend after stabbing him with an arrow in the eye. However, Slade turned out to have survived and resurfaced years later, attacking Starling City as the mercenary named "Deathstroke." Fortunately for Oliver, the villain failed and got locked up in an A.R.G.U.S. facility on Lian Yu, back to where he came from.

Although he is imprisoned on an island far away, the possibility of Deathstroke making an appearance on the show again has not been entirely ruled out, especially now with the latest Twitter post from the Green Arrow actor. Amell tweeted a photo of Deathstroke's mask with the caption, "Hello old friend."

Hello old friend. pic.twitter.com/MWDonZTWNN

— Stephen Amell (@StephenAmell) March 20, 2017

Could this mean that Oliver's nemesis will escape from prison and appear in the present time? Or could he just be appearing in a flashback that would show more of the events that happened on Lian Yu?

According to Cartermatt, there will be no more flashbacks after the current season. So, all the missing information about everything that happened on Lian Yu is bound to be revealed before "Arrow" season 5 ends. This could be where Deathstroke comes in.

Meanwhile, as the season 5 finale comes to a close with just a few more episodes left, showrunner Wendy Mericle revealed to Rotten Tomatoes during a panel at the Paley Festival 2017 that having Prometheus as this season's big bad will bring out an entirely different ending. "I'll just say this: Star City is not in jeopardy," she started. "As we've joked before, there's always a terrorist attack on Star City in May. Not this time. Prometheus is a very personal villain, very psychological villain and the ending will be appropriate to that."

"Arrow" season 5 airs Wednesdays at 8 p.m. EDT on The CW.