The fallen aren’t the darkness because they found the traveler before the humans and when it left it caused their collapse ( I believe it was called the whirlwind). After that the human race found the traveler. Why I say they aren’t part of the darkness is they found the traveler and used the light and if they were the darkness the would have destroyed the traveler. ( There are a few plot holes so take this with a grain of salt, also this is my opinion)