The darkwand was used by Pen to rescue his aunt Grianne Ohmsford from the Forbidding. It helped him track his aunt, and later to track the Moric, the Demon who had been transported out of the Forbidding in exchange for Grianne's imprisonment there.
To make the wand from one of Father Tanequil's limbs, Pen Ohmsford had to give up two of his fingers and the person he loved most, Cinnaminson, whose body was taken by Mother Tanequil while her soul was allowed to roam free as an aeriad, a spirit of the air. Under the direction of the tanequil, Pen carved runes of magic into the darkwand.
The darkwand responded to Pen's needs and helped him on his quest into the Forbidding, guiding him and even keeping a fascinated Dragon at bay. Pen was later able to use the wand to send the Moric, which had disguised itself as Federation Prime Minister Sen Dunsidan, back to the Forbidding. The staff was transported at the same time, at which point it became lifeless.