It was parselmagic, just like the original spell. But where Voldemort's mark had burned through him, shackled his soul, Potter's ... Harry's was like a soft caress, like blankets wrapping around him to shield him from the cold. He felt magic waving them together, and he wondered if he would be able to protect himself from the boy ... young man in the future. While the spell's tendrils were far gentler, they also reached a lot deeper into Severus" heart, body and soul. If he had anything left to lose, he would baulk now, try to guard a part of himself. But that was no longer an option. He had made a decision long ago, holding his best friend's dead body, swearing that he would protect her son. When Potter pulled his hand back, his body shut down.