General:
* Last journey
* Farewell voyage
* Final trip
* Last sailing
* Final cruise
* Final expedition
More specific (depending on the context):
* Death journey (if referring to a person's passing)
* Last voyage home (if returning to a specific place)
* Voyage to the unknown (if referring to a journey with an uncertain outcome)
* Voyage of no return (if the journey is one from which there is no coming back)
The best synonym will depend on the specific context of your writing.