Norman Osborn, director of H.A.M.M.E.R. was uncomfortable with Asgard, the home of the Norse Gods, hovering above Broxton, OK, as it was American soil. Power-mad, Osborn asked the president of the US if he could invade Asgard, to which the president emphatically answered no.
Wooed by Loki, Osborn used Norse God Volstagg as a catalyst. Volstagg is known for wanting to smite foes on his adventures, and, with events put in place by Loki and Osborn, accidentally caused an accident in Chicago, costing thousands of Americans their lives. This was the excuse Osborn felt he needed to invade Asgard; Combining the forces of his Dark Avengers with everything at H.A.M.M.E.R.'s disposal, as well as The Hood's army of supervillains, Osborn tried to overtake Asgard. Steve Rogers, the New Avengers, Nick Fury, Thor, the Secret Warriors, and all other available heroes fought against Osborn's forces.
When Ares, God of War, realized the treachery in Osborn's plot, he turned on Norman, only to be torn in half by The Sentry. Osborn ordered the Sentry to take down Asgard; Destroy the entirety of the Norse Gods' home. Upon seeing his home destroyed, Thor did battle with The Sentry, but the battle proved to be too much for Sentry's fragile mental state, and his dark half, The Void, was unleashed.
Upon Asgard's collapse, Loki immediately regretted his decision to side with Osborn, as he did not think that this was a possibility. He immediately retracted all of the power he'd given to Osborn's forces, and imbued it upon the Avengers. Everybody working together, with assistance of Asgardian magic, was able to defeat the Void, though not without Loki's apparent death.
Bob Reynolds, the Sentry's human personality, saw what he had done. He asked that the heroes kill him, as he was too dangerous. Though immediately declining, The Void began to take over Reynolds' body, again. Thor took the Sentry, and flew him into the sun, permanently ending the danger.