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.
What does Sylar do to Matt Parkman?
Sylar escalates his tormenting of Parkman, using his own telepathy against him to force Parkman to use his powers to save himself.
How does Parkman react to the haunting and tormenting using his own telepathy against him
Matt decides to keep Sylar in his mind, which begins to have consequences when Sylar's control over Matt advances.

Question: What kind of consequences begin when Sylar takes control of Matt?
Self-contained rewrite: What kind of consequences begin when Sylar takes control of Matt?