Guild Wars 2 Halloween 2012
Find Magister Tassi in Lion's Arch. She is located to the North of the Gate hub. Speak with her and you will receive a letter containing instructions, Matter Meter and a Trick or Treat bag. Open up the bag to get your Candy Corn which will be used by the Matter Meter.
Use the Matter Meter near Magister Tassi, then use the skill Etheric Field Scan. Spirit Serene will then appear. Talk with Serene.