Tarkleigh has asked you to defeat the Puppet Mistress. You will find her in The Spawning Grounds which is a side area in The Wetlands zone of Act 6. There is no waypoint in The Wetlands so you will need to go from the Riverways waypoint to the Wetlands entrance. Upon entering the grounds you will need to go down the steps and kill everything there and then come back up where she will be waiting and kill her.
You have defeated the Puppet Mistress. Talk to Tarkleigh for a reward.
Quest Complete - You have defeated the Puppet Mistress. Tarkleigh has rewarded you with a Book of Skill. Use it to gain 1 Passive skill Point.