Damien And The Legion Of Doom
It seemed that this would be the end for Darhk, but the character returned in Legends of Tomorrow, when he was recruited by Eobard Thawne (Matt Letscher) as a member of the Legion of Doom. Eobard, aka Reverse Flash, headed back in time to warn Darhk of his impending doom at the hands of Arrow, and convinced him that there was a better way to gain power. Along with Malcolm Merlyn, Eobard and Damien plotted to reunite the pieces of the Spear of Destiny; a magical artifact with the power to re-write reality, not just time. The trio made their way through history, eventually managing to collect the pieces and reunite the Spear to create their own version of reality: Doomworld.
Although the Legion of Doom had ultimate power in this new world, it didn't last long. The Legends of Doomworld were able to reunite and form a plan to go back in time and stop the Legion from ever getting hold of the Spear. It was a dangerous plan, as this meant going back to a point in history where they had already been, and breaking the primary rule of time travel. Despite this, they decided that the risk was worth it, if that meant stopping the Legion. Together with their past selves, the Legends were able to defeat the Legion, returning reality to its original form (and breaking time, of course).
Eobard was erased from the timeline, and Merlyn was returned to the present day. Sara Lance (Caity Lotz) struggled with her desire to kill Damien out of revenge for Laurel's death, but eventually was able to let him live to preserve the timeline. The Legends wiped Damien's memory, and dropped him back in time to where Eobard picked him up, to live out his original timeline, come to Star City, and be killed by Green Arrow.
Arrowverse Round Three
Which bring us to Damien v Arrowverse round three, with the news that Damien will be back for the third season of Legends of Tomorrow. Producer Marc Guggenheim has confirmed that this is not another time-traveling version, but a resurrection of the dead Darhk:
[Legends of Tomorrow season 3] is a different Damien Darhk than the one you saw last year. This is the post-Arrow season 4. We pick up after he’s been killed. We have a fun way to bring him back.
This week, we learned what that 'fun way' was; Darhk's body appeared in Victorian London, in a tank with a secret occult society attempting to revive him. The Legends didn't realize that he was what they were hunting, though, and initially believed that they were after a vampire. Rip believes that Mallus is the one who wants to bring Damien back, and that he has been brought back in time to be revived during a special astrological event... something that Rip and the Legends are too late to prevent.
He's also not the only Darhk who is back in play. His daughter, Nora Darhk, has also returned to Legends of Tomorrow this year. Nora was last seen as a child around the time that Darhk was killed, and her fate was never revealed on Arrow. It seems that she survived, however, and has been recruited by Mallus with the promise of her father's return. That leaves us with two new villains in the Mallus camp, as it looks like Nora is just as capable of performing dark magic as her incredibly powerful father. Mallus's full plan has yet to be revealed, but it seems like there are lots of familiar faces that will be a part of it.