Absolute skills

I want to get the special ability for the Demon, but I can't figure out why I'm not being given FIRE ARROWS for the ballista in the "Attacks" line. Is it necessary to have Machine Mastery to get them? Or is simply having a regular ballista enough?
I have everything I need in terms of logistics, everything related to Darkness, everything related to Attack, except for "Fire Arrow." I have three special skills and have unlocked the Gate ability, and I'm standing near the mentor, but he still won't give me Fire Arrows...