Well, I wanted an aura that could decrease a flat amount of armor, so I used Devotion Aura as the base skill, planning the armor bonus as negative, but it can't be done, what should I do.
Once you have ticked the preference (Preferences in File Menu, if I'm not wrong) for Allow negative values in object editor, shift + left click on a line to use negative values.
It is. For exmaple in a Lotr game give Aura of Fear to Sauron as an endurance aura. First level -5% attack speed/moviment speed and such. Thx for help i didnt know how to do that too.