It's all told to us in the lightfall campain, weren't you paying attention ? Basically, The Witness is trying to hijack the flower game, before it was light vs dark, but an entity came from an unknown dimension and attacked the Winnower , forcing it to hide, allowing that entity to assume control of the darknes, but what the entity came here for was not darkness, it was just a means to an end, there is a secret the Gardner hides, that another, more powerful force lies beyond a portal it protects, and only the Winnower , aka The Veil , can unlock said portal That's why the witness said no more life(traveler) no more death(winnower) The Witness Corrupted both the light and dark to play a different game According to what Nimbus said anyway