User blog comment:TheDoctor230/Damien Darhk Immortal?/@comment-3423279-20160513042922

Damian Dhark is the former best friend of the Ra's that Matt Nable played, and so has been alive for more than a human lifetime at the very least. He took some of the Lazaras Pit waters when he fled due to League of Assassin shenanigans which he must of used to extend his own lifetime.

Granted he isn't immortal in that he is unable to die, Dhark simply cannot age if he keeps using the Lazaras Pit and possibly that life-force absorbtion magic.