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.
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 that Sylar had sex with Matt's wife Janice while Matt was asleep?
Scared of what Sylar could do to his family, Matt tells Janice the truth, and Janice agrees to let Matt try to exorcise Sylar by leaving the house.
Was Matt's attempt to exorcise Sylar by leaving the house successful?
Sylar appears pained whenever Parkman drinks alcohol, giving Parkman the idea to drink into a stupor. Sylar appears successfully removed.
What happens to Sylar after he appears successfully removed?
In actuality, Sylar simply tricked Parkman into knocking himself out. Now much more powerful, Sylar takes full control of Parkman's body, leaving Matt trapped in his own head.

Question: What did Sylar do once he was in control of Parkman's body?
Self-contained rewrite: What did Sylar do once he was in control of Parkman's body leaving Matt trapped in his own head?