Collects Omega the Unknown #1-10 (1976-77) and Defenders #76-77.
From the publisher:
Marvel's short-lived superstar fought enemies both infamous and obscure, but it took his death to unveil the story of his life! It's demons, depowerment and drama when the Defenders discover the true secret of Omega and his mysterious charge! Plus, the death of a super-villain who, so far, is still dead! When was the last time you saw THAT? Guest-starring Spider-Man (if you look closely enough), the Incredible Hulk, and the Defenders!
Collecting: Omega: The Unknown #1-10 and Defenders #76-77 -- written by Steve Gerber, Mary Skrenes, Scott Edelman, Roger Stern, Steven Grant, and Mark Gruenwald; and illustrated by Jim Mooney, Lee Elias, and Herb Trimpe. Cover by Gil Kane.