Zombies, ghosts, time-lost souls, evil doppelgangers; whatever their true nature, they are legion, they wear the faces of the dead, and they live to kill. The Legion of the Unliving is a recurring association of seemingly deceased warriors conjured by various masters to attack the heroic Avengers. The Legion's various organizers - including Kang, Grandmaster, Immortus and the Grim Reaper - usually have some sway over time and space, so the Legion's membership tends to be drawn from a wide array of locations and time periods ranging from the distant past to the far future. The actual state of the Legion's members is nebulous; its ever-shifting roster has included spirits temporarily resurrected from the dead, beings plucked from time periods prior to their deaths, murderous duplicates of the dead, and various combinations of same.
Some of them retain the original personality of the dead warriors, perhaps even some element of free will; other Legion members are re-created as soulless killers, or as blindly obedient pawns of their masters, or both. Sometimes they replicate the normal appearance of the deceased; other times, they appear as decaying, corpse-like facsimiles of their once-living counterparts. There is even some inconsistency regarding whether all the figures represented in the Legion are actually dead - some of the "dead" beings "raised" for the Legion are later revealed to have never truly died at all. The one common factor seems to be that Legion members have all been believed dead at one time or another, giving them an edge in terms of psychological warfare (the horror of being attacked by dead men walking). This emotional impact was a major factor in Kang's decision to create his Legion of the Unliving, and presumably motivated subsequent Legion creators as well.
The Legion grew out of the ongoing conflict between the time-spanning Kang the Conqueror and his Avengers foes. During his quest for the Celestial Madonna (which turned out to be Avengers associate Mantis), Kang fought his own alternate-timeline counterpart Rama-Tut in a battle that hurled them both to the timeless realm of Limbo ruled by Immortus, yet another alternate-timeline counterpart of Kang (though Kang himself did not know this yet). Forging an anti-Avengers alliance with Immortus (who imprisoned Rama-Tut), Kang recalled how Immortus had once pitted a group of historical figures against the Avengers (though it had actually been Immortus's shape-shifting Space Phantoms replicating those figures). Deciding to improve upon this plan by enlisting more powerful troops, Kang used Immortus's time-manipulating technology to enlist an assortment of warriors from past eras, all dead or believed dead: early Avengers foe Baron Zemo, the legendary monster of Frankenstein, the Ghost of the Flying Dutchman, the android Human Torch, martial artist Midnight and one-time Avengers associate Wonder Man (a foe selected to distress the Avengers in general and the Vision in particular, since the android Vision's artificial mind was based on Wonder Man's brain patterns).
Once the Legion was assembled, Kang betrayed and imprisoned Immortus, transported the Avengers to the labyrinth of tunnels beneath Immortus's castle, and sent the Legion into the tunnels to capture Mantis and kill the rest of the Avengers. During subsequent skirmishes, Iron Man was seemingly slain by the Torch, the Ghost was apparently destroyed while gravely wounding the Vision, and Mantis defeated Midnight. As the conflict wore on, the Legion members began to regain more of their free will. Feeling an instinctive kinship to a fellow artificial being, Frankenstein's Monster and the Torch protected the fallen Vision despite Wonder Man's intense hostility toward the android Avenger, and the inhuman duo soon turned against Kang, whom Thor forced to flee while Vision defeated Wonder Man. Meanwhile, Hawkeye freed Rama-Tut and Immortus, who reduced Zemo to protoplasm. Once the battle was over, Immortus restored Iron Man, the Ghost and Zemo, and sent the Legion members back to their proper times, though not before the Vision and the Torch learned to their surprise that the Vision was a reconstructed version of the Torch.
Years later, the deceased cosmic gamesman known as the Grandmaster conspired with his fellow Elder of the Universe, the Collector, in a plot against the Avengers. Causing the Avengers and their ally the Silver Surfer to die, the Grandmaster and the Collector manipulated the newly deceased heroes into battling each other in Death's realm, distracting the embodiment of Death itself long enough for the Grandmaster to imprison her and usurp her power. Giving the Avengers a sporting chance to stop him, the Grandmaster created five "Life Bombs," each capable of wiping out a full fifth of the universe, and scattered them to the far corners of the universe. If all five detonated, the chain reaction would start a new Big Bang and create a new universe for the Grandmaster to play with.
To guard the bombs, Grandmaster conjured a new Legion of the Unliving, including murderous incarnations of heroes and villains such as Baron Blood, Black Knight (Sir Percy of Scandia), Bucky, Captain Marvel, Death Adder, Dracula, Drax, the Executioner, Green Goblin, Hyperion, Korvac, Nighthawk, Red Guardian, Swordsman and Terrax. In the battles that followed, only one bomb detonated and the Legion was defeated, but Captain America and Hawkeye were the only two heroes who survived. The other heroes - including Black Knight (Dane Whitman), Captain Marvel (Monica Rambeau), Doctor Druid, Iron Man, Mockingbird, Moon Knight, Doctor Pym, She-Hulk, Silver Surfer, Thor, Tigra, Wasp and Wonder Man - were added to the ranks of the Legion; however, before Grandmaster could begin round two of the battle, Hawkeye lured him into a rigged game of chance that distracted the Grandmaster long enough for Death to regain her power and cast Grandmaster out of her realm, restoring all the newly slain Avengers and the Silver Surfer to life in an apparent gesture of gratitude.
Later, when the Avengers opposed Immortus's attempts to enslave the Scarlet Witch, he conjured a new Legion consisting of merciless incarnations of the [[Black Knight (Nathan Garrett), Grim Reaper, an alternate-future Iron Man (Arno Stark), Left-Winger, Oort the Living Comet (a possible-future villain the Avengers hadn't even encountered yet), Right-Winger, Swordsman and Toro (who made the obviously false claim that he had been the "Torch" in the first Legion, perhaps as part of Immortus's new efforts to muddy the true histories of the Vision and the Torch). Oort and Toro struck down Quicksilver and Wasp, but most of the Legion were quickly neutralized, and the entire Legion had faded away long before Immortus was finally defeated.
Drawing power from the demonic Old Ones, including Lloigoroth, with promises of carnage, the undead Grim Reaper sought revenge on the Avengers. He caused a fatal commercial airliner crash and captured a commuter train, transforming some of his victims into decayed yet super-powerful zombie facsimiles of deceased Avengers foes: Amenhotep, Baron Zemo, Black Knight (Garrett), Count Nefaria, Inferno (Joseph Conroy), Nebulon, Necrodamus, Red Guardian, and the Star Stalker. Reaper's Legion nearly destroyed the Avengers, supposedly to avenge the Reaper's death, but his hold over the zombies broke when Vision forced the Reaper to admit that he had killed himself and the Avengers were not to blame. The zombies turned against their master, and the Reaper was seemingly consumed by Lloigoroth for his failure.
Later, the Reaper's brother Wonder Man died again and mystically returned in a ghostly state. This weakened the barriers between life and death enough for Grim Reaper's spirit to return to the earthly plane, mystically enslaving the spirits of Wonder Man and several other deceased Avengers: Captain Mar-Vell, Doctor Druid, Hellcat, Mockingbird, Swordsman and Thunderstrike. This new Legion attacked the living Avengers, but the undead Avengers were freed from the Reaper's control by the Scarlet Witch and helped defeat the Reaper. In the end, most of the undead Avengers returned to their respective afterlives, but Wonder Man was fully resurrected through the Witch's magic, which also restored the Grim Reaper to life after he was forced to admit his love for his brother. Hellcat was soon fully resurrected as well, in another supernatural adventure flowing from these events.
For now, the Legion of the Unliving have all been laid to rest again, though sufficiently powerful villains will doubtless raise more Legions to plague the Avengers anew. In the meantime, a number of Legion veterans have returned to life, such as Baron Blood, Count Nefaria, Dracula, Drax the Destroyer, Frankenstein's Monster, Grim Reaper, Hellcat, Human Torch, Midnight (as the space-faring re-animated corpse, the Midnight Sun), Nebulon, Nighthawk, Red Guardian, Terrax, Wonder Man and possibly Green Goblin (Norman Osborn), though the Torch has since died again.