The Exec. Producer for Justice League, Deborah Snyder confirmed that Steppenwolf will be the main villain in Justice League, though the final casting for the role hasn't been decided just yet.
Steppenwolf is Darkseid's uncle, and also one of his most valued lieutenants. Steppenwolf's been killed and brought back to life multiple times, with the first and most memorable time being after he assassinated Izaya, the Highfather of New Genesis.
Heggra, Steppenwolf's sister was Darkseid's mother, until Darkseid went on to kill her. Steppenwolf's basically known as the undisputed military mind behind Darkseid's army of Parademons.
Who should play Steppenwolf? Check out the deleted scene from Batman v Superman, which shows us our first glimpse of the villain in action, below: