Shrieking Harpy can be purchased with the Ancano's Cunning deck or purchased alone for 100 soul gems.
Shrieking Harpy's power is extremely useful. It can be used to prevent large amounts of damage to you and your creatures. Use it to shackle your opponents strongest creature, or a creature with lethal. This can buy you time to either kill more of your opponents creatures or deal more damage to your opponent directly before they are able to kill your creatures.