Nebulon is the man behind the plan of the Squadron Sinister and it's the villains united versus the Defenders. Plus, a new member joins the Defenders team.
The Defenders are caught in a force bubble by Nebulon. He is about to kill them, but Hyperion (really, a construct of the "real" Hyperion) explains his origin. He came from a micro-world on an atom, which Earth scientists destroyed in an experiment. He came to our world via the intervention of the Grand Master, and fought Thor. Eventually he was stuck in a force bubble and ended up in space, where Nebulon randomly found him. Nebulon was looking for resources to help his people. Hyperion pretended he owned the Earth and sold it to Nebulon for its resources.
Nebulon puts Nighthawk, who originally worked for him but turned against him, in the same bubble as the Avengers, and sent them into space. However, the Defenders focus their powers in one spot and break the bubble. Strange returns them to Earth.
They attack the Squadron Sinister again, but this time Nebulon decides to fight them directly. This time the Defenders put him under enough stress that he reveals his true form, a giant tentacled monster. Nighthawk turns the laser cannon that he created (under duress) to melt the ice cap on Nebulon, and he gets overcharged with energy. An explosion envelops him and the Squadron--as well as Nighthawk.
Nighthawk is dying, so Strange gets the Defenders to agree to give up a little bit of their own life force to save him. The spell works, and Nighthawk revives. He joins the Defenders, but Namor is none-too-pleased at the way Strange teleported him to help without his consent. He flies off and tells them not to call on him again.