From IGN..
Slade Wilson will be introduced in the ninth episode of Smallville this season, called "Patriot." We'll only know him by the name Slade when we meet him, but DC fans of course are aware that this is the man destined to become Deathstroke, an incredibly skilled and dangerous mercenary.
Michael Hogan(Saul Tigh from BSG) will be playing Wilson. He has experience playing a very gruff character, with an eye patch! Should be very interesting.
Apparently
Hogan is currently slated to appear in two episodes, including the eleventh episode of Season 10, which is called "Icarus." After that, we'll see if he'll show up again before the show comes to a close at the end of the season.
In the meantime, you can catch the return of Hawkman (Michael Shanks), on tonight's new episode, "Shield", airing at 8:00 on The CW.