Isn't it obvious? It would be cool if there were some sort of montage where Darhk was sent back and attempted to change the future, only to find himself in the exact same position with the exact same death (only with tiny little differences, like maybe he kills Laurel somewhere other than a prison, kind of like Flashpoint). It would send the message that certain big events will always happen the same way, no matter what (kind of like Rip's futile quest to save his family). But this show is cheesy popcorn movie fun (not always a bad thing) so I don't expect anything that deep or complex. My guess is that the blatantly obvious macguffin, the Spear of Destiny will be used to deus ex machina the Legion of Doom out of existence and back to their proper places in history. It seems lazy (and probably is). But its the best way to get around all of that. A more confusing, but possibly more satisfying, way would be for the Legends to pull off some plan in which they somehow lure Zoom/Black Flash to a past version of Thawne at the inception of his plan and remove him from the timeline at that point, undoing all of the events of the season. But that seems almost as overly complex as killing Vandal Savage in three different timelines at the exact moment. It may be best to simply go with using the Spear to fix everything.