place (oneself) at (someone's) mercy
1. To plead with someone to show one lenience and mercy. I have no excuse for what I did; I can only place myself on your mercy and beg for forgiveness. She threw herself at the court's mercy, arguing that her dependence on drugs led her to commit those crimes.
2. To put control over one's fate into someone else's hand. Anyone participating in the scheme is essentially placing themselves at the government's mercy.