The next morning, Lily woke up quite late. She felt rested. She went to the bathroom to get a glass of water, trying to recover from the strange dream she'd had. She looked at herself in the shabby mirror. For a moment there, she was afraid that the eyes looking back at her would be black. She felt relieved when she saw the eyes were green. Her own. She took a sip from the glass, not diverting her gaze from the reflection.
In her dream, she was told by Voldemort to torture herself. The other Lily wasn't against it. In fact, she commanded her to obey the Dark Lord. The command was too strange for her to comprehend and yet too strong for her to disobey. She felt her hand moving against her will, reaching into her pocket for the wand. But the clothes weren't hers and nor was the wand. She looked at her sleeves in shock. They were black.
"DEMNATIOREM MORTIS CREBRE," she screamed from the top of his lungs.
Stepping into the kitchen, she noticed that the house was unusually silent. Where was Severus, she wondered?
She stepped down into the living room with the intention to sit on the sofa and eat her breakfast in peace and contemplate on next steps… But she saw this.
Severus was lying motionless on the floor. Lily gasped and rushed to kneel beside him.
"Sev!"
He didn't look dead. Occasionally, his body jerked violently, his face contorting itself into an agonized expression.
"Sev!" she cried again, not expecting any answer.
Instinctively, she put her hand on his shoulder, but she shouldn't have done that…
A sharp, electric pain ran through her body. Before she knew it, she found herself lying on the floor beside her friend and when she finally realized what was happening, she could feel she'd been shaking for a while but she had no idea how long. She got on her feet. She knew what it was. Demnatiorem mortis crebre. The curse from her dream.
Gently and with great caution, she lifted him with Wingardium leviosa and transported him into his bedroom. She pushed her tears back, telling herself to remain focused, otherwise she might drop Severus accidentally.
She put him on the bed, took off his clothes with another spell (it took her a long time) and pulled a duvet over him. He looked like he'd been dragged through hell on a thestral's back. Immediately, he curled into a ball and Lily sat down next to him, while being careful not to touch him again.
"What have you done, Sev?" she whispered at him.
Suddenly, it all started to make sense. It all started to come back to her.
He used an Imperius curse, making her do whatever he ordered her to. And then… He must have ordered her to drink a certain type of potion… And he drank it, too.
Polyjuice potion. Yes, that must be it.
They switched appearances. She was Snape and Snape was her. Under the imperius curse, she accepted commands to go to the death eater meeting, acting like him.
When Voldemort inspected Lily, of course he found nothing… That Lily wasn't under the imperius curse. Severus was!
Of course, during the practice of the new curse, "that Lily" must have lost the mental connection to the real Lily, making real Lily wonder what she was doing there and why there was another Lily in front of her and why she was Severus and why she was told to inflict pain on herself…
The "dream" made perfect sense now.
He took the suffering on himself by switching places!
Lily looked him in the face. It was still contorted in pain. "Sev," she whispered again and some of her tears fell on his pillow.
She spent the whole next hour trying to ease his pain with a spell. She tried various calming spells. None of them seemed to be working. Then, she tried an old spell she learned while she was pregnant, with the intention to calm her child. She thought maybe it would work on Severus, as well…
She put her hand over him, palm facing down… And sang the incantation.
"Leniret iste puer, salvum illum de mala somnia…"
It didn't work, either. She didn't expect it to, anyway. Lily got desperate, and so she walked over to the fireplace, taking a handful of floo.