Siege: Dark Avengers (Dark Avengers #13-16)

As night falls on Norman Osborn's Avengers, their darkest hour begins! The origin of the Sentry is revealed. Bullseye commits an unspeakable act of murder. Victoria Hand wonders if she's gone too far. And as he plans the final stages of his Siege of Asgard, Norman Osborn begins to succumb to the evil legacy of the Green Goblin - a mind-altering malady that may just be the thing that propels him to ultimate victory! Win or lose, these are the final hours of the Dark Avengers!

Collecting DARK AVENGERS (2009) #13-16 and DARK AVENGERS ANNUAL #1.