Conversation so far:
What is Haunting Matt Parkman?
Matt Parkman, regretting his actions, is haunted by Sylar.
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?
Matt decides to keep Sylar in his mind, which begins to have consequences when Sylar's control over Matt advances.
What kind of consequences begin when Sylar takes control of Matt?
Sylar has sex with Matt's wife Janice while Matt was asleep.
Does Matt find out about Sylar's actions with Matt's wife?
Scared of what Sylar could do to his family (which Sylar himself alludes to), Matt tells Janice the truth, and Janice agrees to let Matt try to exorcise Sylar.
Was the exorcise successful?
Sylar appears successfully removed as he fades from Matt's mind, and Matt remains conscious long enough to see his partner and Janice return.
What happens to Sylar after he is removed?
In actuality, Sylar simply tricked Parkman into knocking himself out.
Now much more powerful, Sylar takes full control of Parkman's body.

Question: What did Sylar do once he was in control of Parkman's body?
Self-contained rewrite: What did Sylar do once in control of Parkman's body?