Gordian knot:

an intricate knot,devised by King Gor-dius of Phrygia,that Alexander the Great cut with hissword: i.e.a problem of great complexity戈尔迪的结，难解开的结，（比喻）极其复杂的问题。
△H.V.1.1.45:“Turn him to any cause of policy,/ The Gordian knotof it he will unloose，”无论什么政治问题一到他的手里，再难解开的症结都迎刃而解。