It's weird that Superboy's last Legion mission...would have so little Superboy or Legion.

From 1980, Legion of Super-Heroes #259, "Psycho War" Written by Gerry Conway, pencils by Joe Staton, inks by Dave Hunt. 

Lightning Lad, Saturn Girl, and Sun Boy fell victim to a mental attack last issue; but a medical team from St. Croix's arrives to help quickly. Suspiciously quickly. With reason, since they know the attacker, a mental patient that escaped. The patient they call the Psycho-Warrior, actually seems to have escaped from another comic, perhaps an old EC title like Weird Science. (Which, admittedly, I was reading just before this one!) 

Young Rejis Thomak was born on the nightmarish colony planet Bunyon's World, which had resisted four generations of colonists with man-eating plants and animals. Even though it's described as a "hell-planet," Rejis had found love with there, and was leaving with his girlfriend for college on earth.  (I concede a college degree is important, but it's implied they had already been fighting monsters for several years, and were going to return to fight more monsters after college! The money may be good, but I'm not sure what degree would be helpful there.)

Pulled off course by a "pre-nova star," Regis manages to get to an escape pod; but is thrown into the release, accidentally leaving his girlfriend to die.  Utterly crushed, while being treated at St. Croix, Regis crosses paths with recuperating Legionnaire Brainiac 5, and develops a pathological hatred of the team. (Somewhat surprisingly, not due to anything Brainy did!) Sun Boy in particular, for being a reminder of his girlfriend's death; but Regis stole "psych-profiles" on the entire team, which gave him access to their greatest fears.  

With the assembled Legionnaires paralyzed by an "optic detonator," Regis is free to attack Superboy with his greatest fear: the deaths of his foster parents, Ma and Pa Kent.  That hits Supes hard, but he's still able to get Regis to face the sun and his own fears and remorse.  Still, afterwards the incident has greatly affected Superboy, who admits he had previously seen his visits to the future as "a never-never land," that has now become real.  He knows he won't remember the details of the Kents' deaths when he returns to the 20th century, but he would every time he came to the future...and that's why the Legion can't let him come back.  

In somewhat typical Legion fashion, instead of talking it over with Supes, Saturn Girl plants a post-hypnotic suggestion, that he not return to the 30th century.  And he wouldn't for some time...probably not until the Great Darkness War, I think.  This issue was part of a DC house ad, highlighting the split between the Legion and Superboy. The Superboy cover has Ma Kent imploring him to put an extra candle on his 16th birthday cake; and I'm not positive I remember why...