Douven Stahl. As an item of shadow, it seemed to me that this little mirror could stand to have more powers over darkness, as well as a small drawback for delving into these dark arts. The item below also encourages the use of rituals by granting a bonus to rituals with certain key skills. Rituals are such an underused component and factor in typical 4E WotC adventures, I've striven to give them a little incentive whenever I can.
The Mirror of Shadows
The silver mirror is tarnished and when you look into it you find your reflection grey and misty. Suddenly you see your face, but it is like a dark corrupted version, glaring out at you with white skin and eyes shining with malice.
This ancient mirror is a relic from the days of the Bael Turath Empire. It can be used as a focus for magic rituals designed to break the barriers between worlds.
Property: You lose 1 Healing Surge from your maximum total, however you gain a +2 item bonus to skill checks when performing rituals with the Arcana keyword.
Cloud of Shadows + Encounter
Effect: One adjacent ally gains concealment until the start of your next turn.
Covering Darkness + Encounter
Effect: All active light sources within 10 squares of you are suppressed until the end of the encounter. Light sources created after you use this power function normally.