hold hostage

Short definition from Wiktionary

Verb

1. 1
   To hold as hostage: To have possession or custody of (a person) as security for performance against a treaty, a pledge, or a demand, especially now an extra-legal demand.
2. 2
   To hold as hostage: To seek advantage in a situation by threatening the well-being or success of (a person or thing not directly involved in the situation).
3. 3
   To hold as hostage: To sabotage (a computer) with intent to blackmail the owner.
4. 4
   To hold in custody or use for any advantage.