Another Ultimate Edition scene showed Luthor communicating with a hologram-projected Steppenwolf before being captured, and since Steppenwolf is Justice League’s main antagonist, we now know this is who Lex was referring to.
When we left off with Lex Luthor in Batman v Superman: Dawn of Justice, he was languishing behind bars, and the Ultimate Edition of the movie revealed that Batman had arranged for him to be transferred to Arkham Asylum.
Luthor didn’t seem particularly fazed by the Caped Crusader paying him a surprise visit, though, as he started babbling about an unidentified figure coming.