It was part of the material that got removed when the blockbuster underwent major reshoots, and details about it have mostly been vague.
That is still kind of the case, but at least we have a better idea of what was initially going to happen.
It was never expected that Willem Dafoe was going to have a huge role in Justice League — the actor openly acknowledging that the work was setting up for Aquaman — but it was still a surprise when he didn’t appear in the movie.