Stealing the power from a were-something pretty much screams full moon, I think.
Hmm, there was an episode of dr who, where they trapped a werewolf-thingy in a cage made of moonlight that was collected through a telescope and dispersed through a diamond to form the cage. You could do something similar, only collect the moonlight to act as a catalyst for the ritual. Or have mirrors all around to create a circle out of moonlight. The theft of the diamond could be what the PC start investigating. The original diamond was quite large, I think it was part of the British Crown Jewels.