Fortunately, now that we've seen this deleted scene, we won't have to wait for future films to explain it. This demon is a DC baddie named Steppenwolf, a general from the planet Apokolips in the service of the fearsome tyrant Darkseid. (Think Marvel's Thanos, only with a face made out of stone.) Those cubes he's holding are Mother Boxes, like the one glimpsed in Cyborg's brief cameo, which are basically living supercomputers that grant their bearer mystical powers. (Think Marvel's Infinity Stones, only square.)