death knell -------- 丧钟（成语）

英文释义
(IDIOM) The final event before total failure; an omen of coming doom. (NOTE: knell means the sound of a large bell rung in a solemn manner as an announcement of someone death)

例句
The closure of the small town only factory may be the death knell of the local economy.
这家小镇工厂的倒闭也许敲响了当地经济的丧钟。