At (someone's) mercy phrase
Under the control of or controlled by the acts of another, without the power to protect or liberate oneself.
He pleaded guilty and he was at the court's mercy.
Peter was kidnapped and briefly detained by a terrorist group. He was entirely at their mercy.
A little girl was at a mad man's mercy. We have to hurry up.
To have someone in a difficult situation in which you have complete power over them.
Daughter: So I really want all my friends to have a great time. I was thinking it would be a good idea to have some candy out on the tables for people to snack on.
Mother: Well, if you want them to really enjoy themselves, candy is dandy but liquor is quicker.