Steppenwolf dates back to the 1970s, introduced to DC Comics by the great Jack Kirby. He's a member of the New Gods, and as it pertains to the DC Cinematic Universe, he's the uncle of Darkseid, and the leader of the Apokolips military. From what we were able to learn on set, Steppenwolf and his Parademons will be in pursuit of three Motherboxes. These seeds will be sewn by Snyder in the DCEU, as was hinted in Batman v Superman: Dawn of Justice when Bruce Wayne had his Knightmare vision (seen below).
We actually saw a hint at the existence of Steppenwolf in a deleted Batman v Superman: Dawn of Justice scene that Warner Bros. officially released days after the movie reached theaters. If you recall, it showed Jesse Eisenberg's Lex Luthor in the Kryptonian ship that he blackmailed his way onto. He was bathing in a mysteriously dark liquid, and we saw a miniaturized version of a demonic figure that some, at the time, mistook for Darkseid but who actually is meant to represent Steppenwolf.