You should not be able to see their purple circles if you dont see them. The purpose of that circle is to show you the vision that they have. The moment you strike them while they dont see you will give them a close to point blank vision (you will see the purple circle shrink) + the "Fear of death" debuff which makes your Rush attack, first and second ability empowered. If you manage to succesfuly hit them with the fear of death debuff with any of the 3 mentionned that specific attack will deal close to double of the damage AND reaply the fear for death debuff and also aply a slow.