mercy

mercy /ˈmɚsi/ noun
plural mercies
mercy
/ˈmɚsi/
noun
plural mercies
Britannica Dictionary definition of MERCY
1 
[noncount] : kind or forgiving treatment of someone who could be treated harshly
2 
[noncount] : kindness or help given to people who are in a very bad or desperate situation
3 
[count] : a good or lucky fact or situation
4 
old-fashioned + informal — used as an interjection to show surprise

at the mercy of

or at someone's or something's mercy
: in a position or situation in which you can be harmed by (someone or something you cannot control)

to the mercy of

or to the (tender) mercies of
: without any protection from (someone or something you cannot control)