I could be wrong since it was a while ago since I read it, but I'm petty sure they brought up the ban mark when she was acting as a ghost, before the skull dude died and right before they found about the dragons nests.
I think you've got it all mixed up? This was after the dragon's nest incident, that's why ephyr's already transformed into human form in honor of elijey;
and if i remember correctly they were just talking about dimensional magic stuff in that "haunted house" while she was still a "ghost"
If they really did talk about ban marks then i must've missed it, but i really don't recall anything about that. ╮(￣▽￣)╭
The only reason they even mentioned the banned mark was so she could die by it. I don't think its even existed before this moment? What do you guys think? (づ￣ ³￣)づ