forbidden fruit
Refers to something desirable but prohibited, often with a moral or tempting connotation. Originates from the biblical story of Adam and Eve.
To him, she was truly the forbidden fruit.
If you reach for the forbidden fruit, you might end up regretting it.
A direct translation of the English phrase 'forbidden fruit', itself from the biblical account in Genesis. The Japanese expression is a calque, using 禁断 (prohibition) and 果実 (fruit).