The Conclusion
Fantastic Four #416
It starts out with Nathanial Richards warning the team that if they do not flee this fight, they will all die, as he has seen the future. Then, Onslaught manipulates Franklin Richards into sending images, though they are real to the touch, of all of Fantastic Four's enemies to attack them (Annihilus, Blastaar, Dragon Man, Mad Thinker, Malice, Tyros the Terrible and Wizard). They are defeated quite easily with the help of some old friends who show up, as well as some enemies who are allying with the FF. These include Fantastic Force, The Inhumans, Agatha Harkness, Alicia Masters, Namor, Puppet Master, She-Hulk and most notably Dr. Doom!
X-Men v2 #56
Rogue and Joseph/Magneto are about to be killed by a Sentinel, if not for Doctor Doom showing up. He's kind of disgusted with Magneto, and he pops mad shit about Onslaught.
As the Avengers/X-Men prepare for their final strike, Professor X confronts Onslaught on his own, who now has absorbed the powers of Nate Grey as well.
Onslaught: Marvel Universe
Heroes, Mutants & Villains alike gather for the final stand against Onslaught:
The heroes fought valiantly, eventually relying upon the ultimate force of the savage Hulk to break through Onslaught’s armor. However, this all proved to be a ruse, as Onslaught’s armor gave way to reveal pure, malevolent psionic energy, which would consume the planet if not for the ultimate sacrifice of many of Earth’s greatest heroes. The Fantastic Four, Avengers, Namor, Doctor Doom, and Bruce Banner threw themselves into the energy, hoping to contain it. They succeeded, at the cost of all their lives. Nate Grey and Franklin Richards were rescued from the monster, and only the X-Men remained (as mutants could not enter the psionic energy) to mourn the dead and to be blamed for the loss of the world’s finest heroes.