Though no direct explanation was ever given about it, it is assumed Dark Strider is PhantomStrider's "dark side." Dark Strider falls into the TV Trope of "Superpowered evil side." Strider typically transforms to Dark Strider when scared or angry. Dark Strider is particularly common in "creepy" themed videos.
Dark Strider wears a dark blue shirt and has an over-abundance of black eyeliner/paint on his face. He tends to have a more gravelly voice than Strider and generally seems to appear from Strider when he is angry. He is typically more harsh and angry in his temperament than Strider would normally be.
Dark Strider can attack with his saber but he typically uses his hands in abnormally fast slashes to attack instead of using his saber. A black liquid can sometimes be seen smeared down his hands.
Dark Strider's first appearance was from a hidden video "LiveStrider Short" called simply "Dark Strider" where he transforms and cheesily fights about 50 ghosts.Hidden video link:https://www.youtube.com/watch?v=1TqzV7IwFeA
Dark Strider is loosely based of Sora's AntiForm from Kingdom Hearts 2, representing the lingering hatred within Strider.
While transformations seem to have begun as a mistake, Strider appears to have become more fluent over the years at transforming between himself and Dark Strider by choice. Strider returns to his normal self when he stops to close his eyes and take a long, slow deep breath.