Conversation so far:
What is Haunting Matt Parkman by Sylar?
Matt Parkman, regretting his actions, is haunted by Sylar, who claims to have lingered in Parkman's mind after Parkman sucked all his memories and replaced them with Nathan's.

Question: What does Sylar do to Matt Parkman?
Self-contained rewrite: What does Sylar do to Matt Parkman?