SPOILER WARNING: This article contains spoilers for "Batman v Superman: Dawn of Justice."
In the clip, some kind of special forces unit enters the Kryptonian ship Lex Luthor uses to create Doomsday. As they wander the strange, alien halls, they encounter an alien creature, who is holding three gray cubes. While Luthor kneels before him, the creature and the cubes dissolve, only to leave Luthor standing in the Kryptonian sludge with sniper rifles trained on his back.
Since Darkseid is teased throughout several of Bruce Wayne's visions, it's very likely this creature is none other than Steppenwolf. In the comics, Steppenwolf is a general on Apokolips, Darkseid's home world. A member of Darkseid's Elite, the character is an immortal with superhuman strength, speed, endurance and reflexes.
If the creature is indeed Steppenwolf, the cubes are likely Mother Boxes. Mother Boxes enable interstellar teleportation (among various other abilities), which would explain the creature's subsequent disappearance.
Additionally, the deleted scene may explain how Luthor ended up in custody, as the theatrical version of the film never shows how Luthor is captured. It would also explain why Luthor seems to know about Darkseid's impending arrival, as heavily implied by Batman's final scene with the character.