Countdown to Messiah Complex: Avengers Disassembled

Avengers 500-503, Avengers Finale

What Happened:
Scarlet Witch, whose powers have been steadily driving her mad, finally blows a gasket and subconsciously manifests a number of attacks upon the Avengers, crippling the mansion, injuring half the team, and killing three members. When they confront her, Dr. Strange stops her assault, but shatters her mind in the process. Magneto arrives to take his daughter to Charles Xavier for help, and the remaining Avengers, devastated, disband the team.

What Matters:
Scarlet Witch goes nuts
Ant-Man (Scott Lang), Hawkeye, Vision, and Agatha Harkness all die.
Wasp, Captain Britain (Kelsey Kirkland), and She Hulk are seriously injured.
Iron Man is forced to resign as Secretary of Defense of the United States.
The Avengers disband, leaving the mansion in ruins.

Why it's Significant:
Are you kidding? This is where it all starts! While most of the serious events of this story have since been reversed (new Ant-Man, Hawkeye and Vision alive, all injured parties fine, Iron Man director of SHIELD, two teams of Avengers) this directly sets up House of M and its subsequent outcome, M-Day. This is where it all starts, folks.