For episode 100 fans were treated to a short cameo by the villain thanks to some alternate reality hi-jinks but it was just in costume. "Good return, Manu Bennett".
Stephen Amell kicked off the news of Bennett's return by tweeting a picture of Deathstroke's mask with the following caption: "Desperate times call for desperate measures".
More news as we hear it. It does imply that Deathstroke will be involved in the "desperate" struggle between Oliver and Prometheus but doesn't exactly tell us on which side he will fall.
EW has confirmed that the actor will reprise his role as Slade Wilson, otherwise known as Deathstroke. Recently, executive producer Wendy Mericle has revealed that during the remainder of "Arrow" season 5, viewers will be even more shocked by whom he will be working with in order to defeat this season's big bad. Perhaps Oliver goes to Lian Yu, not only to recruit Deathstroke to his cause, but also Black Siren, who we know will be returning in the finale, before joining the series as a regular in season six.
Wilson popped up near the end of Arrow's first season as a fellow castaway on Lian Yu, where he and Oliver bonded and both fell in love with Shado. Now it appears that Slade Wilson will one of the other long returning characters and a presumable ally for Oliver in the battle against Adrian Chase. Guggenheim's post from yesterday (with Bennett himself tagged) shows a scan of a script cover sheet, detailing production notes, captioned - "Guess there's really no point in hiding it anymore...". In the episode, Deathstroke was an adversary of Team Arrow's in the shared dream between Oliver, Thea, Ray, Sarah and Diggle. So ... could a team-up between the Green Arrow and Deathstroke be in the offing to take out Prometheus?