Widely regarded as one of the best story arcs ever presented in modern day comic books, The Great Darkness Saga covered six issues (and assorted annuals and follow-up stories) of the Legion of Super-Heroes in the 80s, written by Paul Levitz and drawn by Keith Giffen.
When Mon-El and his love Shadow Lass take a detour while on vacation to a previously unknown and unexplored planet, they unwittingly disturb the slumber of something very old and very evil.
Several months later, dark "servants" begin rampaging through the inhabited worlds of the 30th century Legion, fighting various teams of Legionnaires to a standstill or worse while making off with artifacts of a mystical or magical nature. These servants appear to be dark mirror replicants of famous superheroes and villians, including Superman, Mordru the Merciless, and even one of The Guardians of the Universe, and their thefts designed to restore their "dark master" to full strength.
Beset by overwhelming power at every turn, the Legion barely manages to hold off the servants on The Sorcerer's World, preventing them from obtaining a very special child, though Sorcerer's World is literally decimated as a result.
Ultimately, the identity of the dark master is revealed as none other than Darkseid of Jack Kirby's New Gods tales. Darkseid invades Mon-El's homeworld of Daxam and enslaves the entire populace, granting each of the three billion inhabitants of that world with the powers of Superman by switching the physical locations of Daxam and Apokolips (the strange world mentioned above). This army then attempts to take over the entire known universe, with only the Legion ... backed up by every superhero they can muster, including the Legion of Substitute Heroes, Superboy, and Supergirl ... to stop them.
And stop them they do, though barely, and Darkseid himself is held off by the combined (and magically enhanced) efforts of Superboy and Supergirl. The "special child" is revealed to be the New Gods Highfather who uses a mother box to turn one of the dark servants into Darkseid's son and ultimate foe Orion, Dog of War, who delivers the decisive blow to Darkseid. Darkseid then withdraws, stating that his time of ascendancy has not yet arrived. The hotheaded Legionnaire known as Wildfire brashly declares total victory over Darkseid, causing Darkseid to curse the Legion, stating "the purest of you shall be the first to fall".
This curse came to pass some time later when Darkseid abducts one of Saturn Girl's and Lightning Lad's newborn children, sending the child back through time and altering the child to become Validus, one of the Legion's greatest foes.