User blog comment:ZombifiedRipper/Possible Chance that Ra's Al Ghul May Appear/@comment-67.86.228.213-20131024172903/@comment-24034641-20131027163241

From what's been released he's only appearing in 3 episodes this season, episodes 8-9 and episode 20 So it maybe episode 20 when be becomes The Flash (as episode 20 is apparently being based on mainly him)

Unless episode 18 is a brief cameo