Wanted TPB: The Time Trapper - Volume 1

Writer: Various
Artist: Various

Publisher: DC Comics (1986 and 1987)
Includes: Cosmic Boy #1-4, Legionnaires 3 #1-4, and Superman vol. 2,#8; Action Comics #591; and Legion of Super-Heroes vol. 3, #37-38

Clipped from Wikipedia: In the 30th century, Cosmic Boy and his girlfriend Night Girl (of the Legion of Substitute Heroes) have returned from a journey to 20th century Earth, where they were attacked by the Time Trapper and found history altered such that Superman never had a teenage career as Superboy.

Time Trapper recalls how he created a "pocket universe", with its own Earth, its own Krypton and its own Kal-El. In this pocket universe, Kal-El became Superboy at the age of eight, and it is to this universe that the Trapper has directed the Legion whenever they have travelled through time.

Superboy explains to the Legionnaires that the Time Trapper protected Earth in his era from destruction during the Crisis, and promised to keep it safe in return for Superboy's cooperation in defeating the Legion.

When they find the Trapper, they engage him in battle, inadvertently smashing the machine that protected the pocket universe Earth from the effects of the Crisis. With the red skies and antimatter returning (and with Brainiac 5 unable to repair the machinery), Superboy replaces the damaged unit with his own body. The gambit works and the Earth is saved, but the Boy of Steel is gravely weakened. With the Trapper having now made time travel perilously unsafe, Superboy flies the Legionnaires back to the 30th century, carrying the Time Bubble himself. Shortly after their arrival, Superboy dies in Mon-El's arms. The entire Legion mourns his passing, remembering him as "the greatest hero of them all."

The aftermath continued in...The Time Trapper - Volume 2

Dork Note: All I can say is ...any storyline, which ends with Superboy dying should be collected!