Maybe the Domious (spelling?) glyph is the only thing the order has to defeat Drac at that time, but is tainted by a curse that turns whoever uses it evil. In that case, Shanoa would have to "defeat the evil". Or the Domious glyph kills whoever uses it and Albus didn't want Shanoa to die using it.
This is just a theory though. We will just have to see when the game comes out in a month and a half.