In English, "dawn" only occurs in the morning, so when used metaphorically it always refers to the advent of a thing. The word you're looking for is "twilight", which when used metaphorically means the end or decline of a thing. Thus Nietzsche's Götzen-Dämmerung is translated Twilight of the Idols.
There's also the word "dusk", which is the opposite of "dawn", but it tends not to be used metaphorically.
