For those who can not wait for tonight here are some spoilers from reviewers who have seen the episode

Nathan dies at Sylar’s hands. Danko has been framed for murders by Sylar (ie Sylar impersonating him and leaving evidence). Hiro has a stroke due to the return of his power. Peter gains all of Sylar’s powers. Tracy returns as a water based killer. (a la Hydroman) To save everyone Parkman convinces Sylar that Sylar is Nathan.