If you're going to assemble the greatest superheros on your earth you're gonna need a threat to fight. So what - or who - is the threat that the Justice League will face in Justice League? Is it Darkseid, who was heavily hinted at in Batman v Superman: Dawn of Justice? Is it an evil Superman, as also heavily hinted at in Batman v Superman: Dawn of Justice? Or is it... a minor character who only showed up in BvS' deleted scenes?
Yup, it's Steppenwolf. When the deleted scene of Lex Luthor meeting a monster in his Kryptonian warm bath first popped up online the internet thought that monster was Steppenwolf. I couldn't believe it - the monster barely looked like Steppenwolf, and in fact his whole aesthetic was nothing like that of the New Gods, the corner of the DC Comics universe from which Steppenwolf arises. But I was wrong! Boy was I wrong.
In the comics Steppenwolf is the uncle of Darkseid and the military leader of Apokolips. He has a cool energy axe and rides an even cooler hoverbike. I assume those elements will not make it through Zack Snyder unscathed. In terms of the coolest characters from the Fourth World books he's pretty low on totem pole - he was once killed by Clock King, fer chrissakes In the more recent "New 52" continuity of DC, Steppenwolf actually sees some action and invades Earth - I'm assuming that's the run they're basing Justice League on, at least lightly. On some levels that makes him a fairly blank slate for the movies, and keeps Zack Snyder's hands off cool characters like Granny Goodness and Glorious Godfrey and Orion and Metron and Big Barda and Mister Miracle. I'm pretty okay with them burning Steppenwolf off on what I hope is the last Zack Snyder film in the DC Universe.
In Justice League Steppenwolf has a contentious history with Darkseid, which could be why he's invading the Earth. But stopping him - and, I'm assuming, his Parademon hordes - requires more power than the members of the Justice League can muster, and so they must turn to a dead friend for help...
Darkseid is a presence in the movie, and fully appears at the end of the film, but don't expect him to do much. The other DC solo movies between Justice League and Justice League 2 take place in realish time - ie, The Flash takes place after Justice League - so whatever evil plan Darkseid has will wait until Justice League 2 to be revealed.