trump card

noun2

1. •
   A playing card, of the trump suit that is held in reserve until needed to win a trick.
2. •
   (figurative) Something used to obtain an advantage, sometimes unscrupulously.

   • Desperate now to leave, Felix played what he believed was his trump card. ‘You’re forty-whatever. Look at you. You’re still living like this. I want to have kids. I want to get on with my life.’ — 2012