I feel like the darkness of betrayal is Nim/Lorna, since she betrayed her friends and even turned some of them to stone. Ren isn't really evil, just misguided, though I guess you can say the same for Nim. They were both manipulated by Crescent, yet Nim has done more evil things than Ren. The only problem I see with Nim being the darkness is that Damien claims she isn't human, and even calls her an "it", implying she is something akin to a demon.