The Twist: In the climax, Nina believes she gets into a fight with Lily in her dressing room and stabs her with a shard of a mirror. She hides the body and performs the Black Swan act flawlessly, physically transforming (in her mind) into a bird.
The Reality: After the act, she returns to her room to find the 'body' gone. The real Lily knocks on the door to congratulate her, alive and well. Nina realizes she never fought Lily; she fought a hallucination of herself and stabbed her own stomach.
The Ending: Despite the mortal wound, Nina insists on finishing the ballet. She dances the final act as the White Swan, who commits suicide. She throws herself onto the mattress off-stage. As the cast and Thomas gather around her, they see the blood spreading. Nina does not ask for help; she simply stares at the lights and whispers, 'I was perfect,' before the screen fades to white. It implies she likely dies, having sacrificed her life for that one moment of perfection.