From my oldest son.
The Dagger of the Infiltrator is a +3 weapon of nondescript appearance. When it is on your person it allows any Change Self, Alter Self or similar disguise illusion (from a spell or item) to perfectly imitate the look and voice of the last person killed by the dagger. Note that the dagger does not grant you the ability to generate an illusion.
Graeme slipped over the wall without even a whisper of sound then crouched in the shadows near the gatehouse. As the Lieutenant of the guards stepped around the corner to begin his rounds the master assassin killed him with his Dagger of Infiltration.
After hiding the body in a nearby granary Graeme used his Ring of Spell Storing to cloak himself with Change Self. Now a perfect likeness of the lieutenant he headed toward the keep and the Duke's quarters.