One's Achilles' heel（致命弱点）

Weak or vulnerable point; fault, esp in sb's character, which can lead to his downfall.

致命弱点；（尤指某人个性中可导致一败涂地的）缺陷。

关于Ahilles：

【希神】阿基里斯（除脚踵外，浑身刀枪不入）

When Ahilles was an infant, his mother tried to make him immortal by bathing him in a magical river.
当阿基里斯还是婴儿时，其母亲为了使他永生而在神秘河里为他洗澡。

请看例句：

Vanity is his Achilles' heel.

虚荣自负是他的致命伤。