When Damien Darhk is fighting Thea, he specifically mentions Malcolm Merlyn, and even knows that he is the knew Ra's al Ghul. Later, when Thea is talking about what happened, he looks at least a little concerned by the mention of Darhk. Obviously the two have both been involved in the League of Assassins, but is there more to their history than we currently know? It wouldn't be too much of a surprise if their paths had crossed, but it's going to be interesting to see if that develops any further.