under (someone's) spell
Influenced or controlled by someone because one finds that person fascinating, enchanting, or seductive. Our son never acted out like this before he fell under that new student's spell. Ever since I fell under Susan's spell, our relationship has progressed much more quickly than I would normally be comfortable with.
under someone's spell
Fascinated or influenced by someone, as in I think he has our daughter under his spell. This idiom derives from the literal meaning of spell, "a word or formula that has magical power." [Mid-1800s]
under someone's spell
so devoted to someone that they seem to have magic power over you.