A Sway is a magical artifact that can take away a Witch's power and can be used to transfer the power to a human.
Sixteen years ago
The artifact was once charmed by Ethan Conant and given to Eben and the Witch Hunters so that they would be able to kill John Blackwell. If they had succeeded, Ethan thought he could have been with Amelia Blake. However, the Witch Hunters used the item to lure most of Ethan's Circle members into a trap to kill them.
When Adam Conant and Nick Armstrong took up ownership of the Abandoned House, they took and hid anything that could be potentionally dangerous to the Circle. The Sway, due to its abilities, was judged dangerous and Nick hid it in a box in his closet. When John Blackwell returned, he searched for the Sway in order to discover who was the Witch who betrayed his Circle at the Boatyard, but Cassie refused to give it to him. John later discovered that Ethan was the traitor sixteen years, but he also guessed that Ethan could not lend magic to Eben anymore now, because his magic was stripped by the Elders.
The Circle found out that Nick gave his magic to the witch hunters. They resurrected him and Nick used the sway to give them his powers.