The carriage carrying Lucifer, Berde, and the brain finally took off after the brain successfully used the magic they needed to summon the horses once more.
Well, it was no brainer that the horses would begin to pull the carriage right away after being summoned by the brain.
Of course, now that they were moving, the brain...just remained standing still on where it was located before, and Berde was keeping an eye on the brain and Lucifer.
Even though they said that they would travel together and become 'friends,' that does not really mean that Berde would just trust Lucifer that he would not kill them or anything of the sort.
Naturally, since Berde is being able to do something like this under the brain's supervision, it means that the brain is allowing Berde to do something like this.
After all, even the brain recognizes that it would be stupid for him to leave Lucifer unguarded in this place.