Wiktionary
at the mercy of
a. (context idiomatic English) In the power of; defenceless/defenseless against.
Usage examples of "at the mercy of".
Bullen first found himself in the water, then dragged from it into a canoe, and a moment later a helplessly bound captive at the mercy of an enraged foe.
Sometimes I was not so sure but that I should have been more contented to know that Dian was here in Phutra, than to think of her at the mercy of Hooja the Sly One.