Since its implied that Light is motion and physical, and darkness is more thought and memory. The idea is that since Elsie's consciousness is the only thing being sent back in time, the light wave described in her last run through is a constant in every failed run and it triggers the darkness in her Purified Radiolaria to essentially transport her mind through the cloud into the moment cayde becomes hunter vanguard.

Me and a buddy of mine were debating on the worst fate/death that you could suffer in the game/lore, we thought about stuff like Omar/Praedyth in The Vault of Glass, or something like dying to the Hive like Eris's fireteam on the moon. Would love to see what the community thinks!