Is there a word for a promise so solemn that the only way you would fail to keep it is if death stopped you?

I tried a reverse dictionary, but just guessing what to search there isn’t easy, so I am unsure whether there is a word for such a level of commitment or not. If there is such a word, it’s one I find lacking in my current vocabulary, as I sometimes need to express the thought and cannot find a word. I’d be indebted to anyone who can come up with one that’s already in the dictionary, not just made up today. :-)