To be Forsworn is when a practitioner breaks an oath or something of that nature and loses the protections given to every human. It allows Others to do whatever they wish without the interference of the Seal of Solomon established by Suleiman, the Sorcerer King. Usually the practitioner doesn't realize they've lied until they try to use their power and find it gone.
↑The oldest of them made agreements in times well beyond us, to guarantee safety and maintain a kind of peace. Foremost among these agreements is truth. Should you lie, you may well forfeit your power for a time. Break a promise or an oath, and you will be forsworn, and you will be stripped of every protection afforded to even the common, ignorant people that decorate this Earth. - Excerpt from Bonds 1.3
“I’d kind of expected a… clap of thunder?” I said.
“Barring the exceptional moments of idiocy, such as the breaking of an oath, you typically only discover what you’ve done when you reach for power and find it gone.” - Excerpt from Damages 2.4