Great Darkness Saga
The Great Darkness Saga is a storyline by Paul Levitz and Keith Giffen published in 1982 in Legion of Super-Heroes" #290-294. Easily the most ambitious and encompassing storyline of the Legion up until that point, its main plot consisted of the Legion's struggle to contain and defeat Darkseid as he reawakens and consolidates his power in the 30th century.
His main weapons are initially a small set of modified clones of various superbeings of the past, including Shadow Lass' ancestor Lydea Mallor (later to be featured in issues of L.E.G.I.O.N.), one of the Guardians of Oa, Superman and his own sons Orion and Kalibak, but the high point of the story involves his learning of Daxam's existence and mind-controlling its whole population to fight in his stead.
Even after the plot was resolved, it still had consequences for the Legionnaires, particularly Lightning Lad and Saturn Girl who had to deal with Darkseid's manipulation of their son.