From Longman Dictionary of Contemporary Englishbe in somebody’s powerbe in somebody’s powerliteraryCONTROL to be in a situation in which someone has complete control over you → power
Examples from the Corpus
be in somebody’s power• He'll do whatever I tell him to do. He's completely in my power.• He is worried his son will be entirely in his ex-wife's power.• I shall be in his power.• Data from the 1990 census indicate exactly how crucial higher education is in determining earning power.• It was in their power to make and break military reputations; men gave of their best in front of them.• I most heartily wish it may be in my power to serve and save so much innocence, beauty and merit.• They were in her power and at her discretion in all ways - how she loved them.• They were in the power of these dragons.• We must do whatever is in our power to convince them not to marry until they are old enough to know themselves. 