Even though Liam Neeson publicly said he'd be open to reprising his Batman Begins role as Ra's al Ghul, the CW instead went with Riddick actor Matt Nable for Arrow's Season 3 villain. Neeson was probably never in the conversation simply to keep the TV canon away from any ties to the movies plus I'm guessing Nable came with a smaller price tag.
It's unknown how many appearances al Ghul will have during the season we do know his first will happen in episode four entitled 'The Magician.' His name was mentioned often last season in association with the League of Assassins. According to the release al Ghul is "described as proud and unforgiving, Ra's is a ruthless strategist, a master of martial arts, and a shaper of history. He carries the wisdom of the ages, and protects some of its greatest secrets."
Stephen Amell helped in making the announcement by going to Twitter:
Please welcome Matt Nable to Arrow. You know who he's playing. pic.twitter.com/f0I7tmujiD
— Stephen Amell (@amellywood) September 4, 2014
New episodes of Arrow return on October 8, 2014 on The CW.