The Dark Flash is initially introduced after pushing the main Barry Allen out of the Speed Force after he saves his mother from being killed. He is the reason Barry ends up in the 2013 alternate timeline and how he exists in the first place. The Dark Flash is a looming presence throughout "The Flash" and fully introduces himself after the main battle between General Zod, the army, and the heroes of the film ends with the impending death of the universe. Dark Flash reveals he's actually Barry's doppelganger who had been working alongside Barry, Batman, and Supergirl, created by a paradox after trying to save his own world countless times.
The Flash's continued attempts to stop General Zod and his army resulted in him being impaled with spikes from the wreckage of a Kryptonian ship. Viewers see his earliest form when the doppelganger Barry attempts to fix the timeline after the initial loss of watching his world die, leading to him getting impaled by similar spikes seen on Dark Flash's suit. While doppelganger Barry only has a few spikes protruding through his body, Dark Flash has been completely covered by them, serving as a visual identification of how his attempts to fix the past corrupted him.
The timeline crashes around the three Flashes, but after Dark Flash tries to kill the main Barry, his 2013 lookalike gets in the way and is impaled by his future self. As a result, Dark Flash fades from existence since he killed a younger version of himself, and the timeline begins to heal. Dark Flash's death is the event that allows Barry to return to his own timeline, which isn't exactly the same as it was when he first left.
