Ascians

[CyrilLucifer]

I wouldn't necessarily say that we don't have anything to verify the theory that Xth represents the Ascian they serve. Chalice's dialogue has lots of watery puns, despite Lahabrea being fire-based in XIV. With higher servants not matching the master's element, but rather his "trigon", that absolutely puts 12th under Pisces, Cancer, or Scorpio as well for reasons I edited into my post up there. It doesn't completely cement it, but it does add to its defense.

Edit: And this is, of course, ignoring that they openly say they serve Lahabrea.