What is another phrase that means the same as a nail in coffin?

Here are some phrases that convey a similar meaning to "a nail in the coffin," indicating something that seals the fate of a situation or makes it irrevocably worse:

General Negative Impact:

* The final straw

* The coup de grâce

* The kiss of death

* The death knell

* The last nail in the coffin (slightly redundant but emphasizes finality)

Specific to Relationships/Situations:

* A deal breaker

* The end of the road

* The last straw

* The point of no return

* The breaking point

Figurative:

* The hammer blow

* The fatal blow

* The knockout punch

The best phrase to use depends on the context and the specific situation you are describing.