Oesterheld finished the series in 1959 and moved on to do other things, such as a comic book biography of Che Guevara, a horror/sci-fi/mystery series called Mort Cinder, and his wife. (Just saying -- dude had four daughters.) By the time Argentina fell into one of those pesky CIA-backed dictatorships in 1976, Oesterheld was 57 years old and had every excuse to wind down his career and spend the rest of his days making cameos in telenovelas, like a Latino Stan Lee. Instead, he brought back El Eternauta, only with a few changes: The setting moved to a totalitarian future, and the protagonist became a revolutionary leader who urged the people to rise up against the dicta- uhh, alien invaders. The Argentinian military wasn't fooled by this subtle symbolism, though, and they started asking around for Oesterheld, so they could have a little chat with him (the kind that involves pliers and your testicles). Oesterheld went into hiding, but kept writing the comic.
Aaaand, this is where our story turns soul-crushingly sad. Oesterheld's daughters (25, 21, 19, and 18), like countless other college-age kids associated with the Montoneros guerrilla movement, were taken one by one by the military and never seen again. Their husbands were picked off, too. Their infant children were temporarily "relocated" -- a thing the military did back then. Oesterheld kept writing. In the comic, the "Hector Oesterheld" character watches in horror as everyone he knows is murdered by the aliens.
At some point in 1977, Oesterheld himself was caught and became one of the between 10,000 and 30,000 people the dictatorship made "disappear" for political reasons ... but not before turning in the script to the last chapter of El Eternauta II, which was published when he was probably enjoying the hospitality of some clandestine detention center. They killed Oesterheld, but his message lives on through the endless reprints, translations, and crappy pirated editions of his comics. Still, good thing Argentina's government doesn't murder people who disagree with them anymore, huh?