While this isn't a huge spoiler, and will undoubtedly be in promos before the episode if it's true, if you don't like knowing this kind of stuff, now is your chance to leave.
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MTV.com is reporting that Manu Bennett has been enlisted to play Slade Wilson once more in Season 3, Episode 14- roundabout where the filming is up to now, if you've been following it, which would put his re-entrance right after the promised Black Canary trilogy (or so it's been unofficially dubbed).
Not much is known on his role, but if it's only for one episode, it's possible we'll only see him in his cell- indeed, perhaps breaking out of it-, in a flashback or he could pop up in modern day. I'm hesitant to say that Deathstroke is returning, because it may just be Slade, without the mask and gear.
What this does mean, though, is that he's open to being in the rest of the season. Nyssa al Ghul was first introduced in Episode 13 of Series 2, and she reappeared in the finale, flanked by the League of Assassins- Slade's inevitable re entrance, teased at Comic Con, where he is yet to fulfil his promise, may follow a similar path.
Don't expect him to be the main villain, though, at least not for this season. The producers have reitterated multiple times that "Ra's is the Malcolm or Slade of this season...quite unlike any villain we've had."
Thanks to comicbook.com for bringing this to my attention!
EDIT: Manu Bennett reposted the link reporting it to his Facebook page. If that's not solid confirmation, I don't know what is.