During SDCC’s “Warner Bros. Television presents A Night of DC Entertainment” panel, it was revealed that Ra’s al Ghul will be the main villain of the third season. To accompany this news they released a new trailer for season 3 of Arrow which adds footage of Ra’s al Ghul. Ra’s al Ghul has yet to be officially cast but castings are underway.
Ra’s al Ghul has been teased before on Arrow as far back as season 1 and even more so in season 2. The league of assassins were a major part of season 1 and 2 and are responsible for training Sarah Lance and Malcolm Merlyn, and even bringing the latter back from the dead.