What is that purple flower on Teen Wolf (2011)?

On the second episode of MTV's Teen wolf (2011), when Scott and Stiles
go to Derek's house to find the other half of the body they find a
wolf.
Then Stiles sees a purple flower and starts to pull it from the
ground, forming a red circle.
then the wolf turns into the other half of the body

what is that flower???? what does it do??