My sentence is:
There is a rule that you must come to training unless....?
I need something like 'pigs flying outside the window' but more formal.
If you wanted to be literary, you could say,
There is a rule that you must come to training unless Great Birnam wood to high Dunsinane Hill shall come.
It sounds formal, anyway.
Not really answering the question, here, but if you say something like "should something bad happen to X..." and then say "knock on wood" (implied that it's in brackets, as an aside, and probably written in brackets in non-fiction; not sure if you would write that part in brackets in prose, but my editing teacher would say that's grammatically incorrect, most likely). Then you actually rap your hand on a piece of wood for good luck. It dispels the bad luck that would, superstitiously, befall X by you saying it.
To answer your question, I personally say "Apocalypse pending". Because I write a lot of apocalyptic fiction.