Alter Aeon Online Help

Keywords are:  'thief skill shadow mastery'

Skill: shadow mastery           Lvl 30 Thie  (28%)
                                (critical)   Requires: cloak of darkness
                                Group: Thief shadow disciplines

Shadow mastery is a passive skill that allows the thief to use shadow skills
more often, with less danger from the prolonged exposure to the shadow plane.

If you know shadow mastery and monster lore, you will have a better chances
of assessing the vulnerabilities and resistances of shadow creatures.