hold (someone) for ransom
To kidnap someone and keep them until someone else pays a demanded sum of money (a "ransom"). The kidnappers are holding my son for ransom—I'll pay any amount they want!
hold someone for ransom
to demand money for the return of a person who has been kidnapped. The kidnappers held me for ransom, but no one would pay. We will hold Timmy for ransom and hope that the police don't find us.