The Witchcatcher is a collar-like tool used to catch a witch, and compel them to obey the user's commands.
It was used by Dean Winchester and Crowley to capture Rowena and force her to perform a spell to send Lucifer back to his Cage.
Overview
A witchcatcher is a device used to restrain witches, most were thought to be destroyed after the Inquisition. However, Billie, who later started working with Crowley, was able to get her hands on one of these.
The witchcatcher is a large iron collar attached to a leash with seven spikes affixed to the inside of the collar. Once affixed to a witch's neck, the witch is forced to do the bidding of whoever is in control of it.
The witch bound to the collar is incapable of removing it, though it is presumed anyone else can take it off, as shown with Lucifer. The spikes on the collar appear to leave no marks on the witch's neck.