under (also beneath) the foot (or feet) of: under the power or control of, or in a state of subjection to, a person; at a person's mercy or absolute…

at (also †in) the mercy of: wholly in the power of, at the discretion or disposal of; liable to danger or harm from; exposed to the consequences of.

Chiefly Scottish. in (also at) (a person's) reverence, in (also into, under) the reverence of: in someone or something's power or control; in the…

in a person's lurch: in his power. Obsolete.

Law in later use. at (also occasionally †in) mercy: that is absolutely in the power of, or at the disposal of, a victor or superior; liable to…