She can be brutal, but what inspires fear isn't always brutality. It's unpredictability. Some of Entertainment's greatest villains (The Joker is a great example) weren't people who could kill you viciously or even threaten you well, although most if not all of those characters could; they made you fear the fact that what they might do to you could be worse than death. Ghost I had to tone back a bit, because one of the things I was gonna have her do, to showcase that there were no lines that mattered to her, was I was going to have her abduct and experiment on Baichi's sisters with cybernetics.
A villain like Tiamat is good as a villain, but she doesn't quite bring the terrifying fear factor of unpredicatability to the table, at least for me. I do rate her at 7/10, because she is intimidating, terrifying and unsettling in her own right, just not enough to give me chills. Still a great villain that I do like reading about.