I'm looking for the word (either noun or adjective) to describe something that happens in the last minute but is so important and precious that it reverses the whole seemingly set situation.
2 Answers
A surprising and important change of such kind is called a plot twist:
plot twist
(noun) A radical change in the expected direction or outcome of the plot of a novel, film, television series, comic, video game, or other work of narrative.
Revelation ?
a. The act of revealing or disclosing.
b. Something revealed, especially a dramatic disclosure of something not previously known or realized.
c. A sudden insight or idea: "I'd had the idea to dig up Dad's coffin ... I was lying in bed and I had the revelation, like a simple solution to an impossible problem" (Jonathan Safran Foer).
http://www.thefreedictionary.com/revelation
"Last night's revelation has stunned everyone."