Shadowflight is an ability given to War by the demon Samael early in Darksiders and is required before traversing The Twilight Cathedral and the Iron Canopy, as well as being used all throughout the game. After double jumping, you can then hold down the jump button and you will grow a pair of Shadowy Wings, enabling you to glide. When you glide over certain soul currents, you will flap your wings and gain considerable height in the air. War cannot glide for long - after a few seconds the wings will disappear, causing him to fall.
Also, strangely in War's shadow when he uses Shadowflight it shows the wings as whole angel wings rather than a mirror of the wings themselves.