'Arrow' season 5 episode 9 spoilers: Prometheus makes deadly move against Team Arrow; Stephen Arnell teases 100th episode

Prometheus makes a deadly move against Oliver (Stephen Arnell) and the team in the upcoming episode of The CW's hit superhero series "Arrow" season 5.

Titled "What We Leave Behind," the show's 9th episode will see Team Arrow fight against Prometheus again. According to the official synopsis, Prometheus attacks Curtis (Echo Kellum), and his latest move puts Team Arrow in a difficult situation. Oliver realizes that Prometheus already knows all of Team Arrow's secret identities. To make matters worse, he is planning to come for them one by one. Elsewhere, Felicity (Emily Bett Rickards) and Detective Malone (guest star Tyler Ritter) discover one clue that ties Prometheus to Oliver's past.

Before the said episode, "Arrow" will be airing its milestone 100th episode next week. It also happens to be the crossover featuring four of The CW's superhero shows. Speaking with TV Line, Arnell shared his thoughts on what makes the upcoming episode special.

"It is such a radical departure from anything that we have ever done on the show," he revealed. "We utilize the fantastical elements that we bring in on 'The Flash' and carry through to 'Legends of Tomorrow' with the present-day storyline to create something... that is not a flashback [or] a flash-forward. It's something else entirely."

Additionally, the episode will also give Oliver a chance to get a sense of closure with his dearly departed loved ones. It turns out that his parents will return as he enters a different timeline.The actor hinted that it's one of the most emotional moments of next week's "Arrow."

"There [is] a very therapeutic moment for my character towards the end [of the 'Arrow' episode] where I get to say goodbye to people that I never got the chance to say goodbye to," Amell added.

The fifth season of "Arrow" airs every Wednesday night at 8 p.m. ET on The CW.