Umbriri Utility 12Concealing Umbra

You exert your will over the stuff of shadow and conceal an ally in pure darkness. No light reveals your friend now.

Encounter        Arcane, Illusion, Shadow
Standard Action      Close burst 5

Target: One ally in the burst

Effect: The target becomes invisible until he or she attacks, or until the end of your next turn.

Published in Forgotten Realms Player's Guide, page(s) 67.