At the mercy of

compassionate or kindly forbearance shown toward an offender, an enemy, or other person in one’s power; compassion, pity, or benevolence:
Have mercy on the poor sinner.
the disposition to be compassionate or forbearing:
an adversary wholly without mercy.
the discretionary power of a judge to pardon someone or to mitigate punishment, especially to send to prison rather than invoke the death penalty.
an act of kindness, compassion, or favor:
She has performed countless small mercies for her friends and neighbors.
something that gives evidence of divine favor; blessing:
It was just a mercy we had our seat belts on when it happened.
at the mercy of, entirely in the power of; subject to:
They were at the mercy of their captors.
Also, at one’s mercy.
noun (pl) -cies
compassionate treatment of or attitude towards an offender, adversary, etc, who is in one’s power or care; clemency; pity
the power to show mercy: to throw oneself on someone’s mercy
a relieving or welcome occurrence or state of affairs: his death was a mercy after weeks of pain
at the mercy of, in the power of
n.

late 12c., “God’s forgiveness of his creatures’ offenses,” from Old French mercit, merci (9c.) “reward, gift; kindness, grace, pity,” from Latin mercedem (nominative merces) “reward, wages, pay hire” (in Vulgar Latin “favor, pity”), from merx (genitive mercis) “wares, merchandise” (see market (n.)). In Church Latin (6c.) applied to the heavenly reward of those who show kindness to the helpless.

Meaning “disposition to forgive or show compassion” is attested from early 13c. As an interjection, attested from mid-13c. In French largely superseded by miséricorde except as a word of thanks. Seat of mercy “golden covering of the Ark of the Covenant” (1530) is Tyndale’s loan-translation of Luther’s gnadenstuhl, an inexact rendering of Hebrew kapporeth, literally “propitiatory.”

compassion for the miserable. Its object is misery. By the atoning sacrifice of Christ a way is open for the exercise of mercy towards the sons of men, in harmony with the demands of truth and righteousness (Gen. 19:19; Ex. 20:6; 34:6, 7; Ps. 85:10; 86:15, 16). In Christ mercy and truth meet together. Mercy is also a Christian grace (Matt. 5:7; 18:33-35).

Also, at someone’s mercy. Subject to the power of, helpless against, as in The captured rebels were at the mercy of the army commander. [ Late 1500s ]
Without any protection against, as in On top of Mount Washington we were at the mercy of the elements. [ Late 1600s ]
see: at the mercy of