Like many of the monsters in the Castlevania series, Legion can trace its beginning back to Christian stories. Specifically, in this case, the being known as "legion" comes from a story of Jesus who found very disturbed man. When asked his name, the man stated he was, "Legion." Jesus found that the man wasn't possessed by just one demon but many, a conflagration that had taken over his form and were driving him mad. Performing an exorcism, Jesus drove the demons out of the man's body and, at their behest, into a herd of pigs who then promptly ran themselves into a river and drowned.
While the name comes from the demon (and likely that story derived the name of the demon from the name for a group of roman soldiers, a "legion", as many of the stories told in the Bible were really about the oppression Christians faced at the hands of the Roman Empire), the actual form of the demon as seen in the Castlevania series likely can be traced to it's other name, "Granfaloon". That name can be traced back to Kurt Vonnegut's 1963 novel Cat's Cradle, a work that describes a "granfaloon" as "a group of people who affect a shared identity or purpose, but whose mutual association is meaningless." That definition was meant to be satirical, although in the Castlevania series it has taken on a much more literal form.
The association of these two beings, that of the demon Legion and the presence of Granfaloon, as certainly rewritten the being for most fans of the series. The Legion we know is the one we think of, "of the many, the one," and yet it doesn't come from mythology by science fiction. It's a weird and interesting origin for this iconic creature that feels like a permanent part, now, of the Castlevania series.
Fighting Against Legion
When Legion first appears in the series, as the being Granfaloon, it made quite the impression as one of the giant, screen-filling bosses Alucard came across in his exploration of the two castles in Castlevania: Symphony of the Night. Appearing in a form that would become his standard, Legion was seemingly nothing more than a mass of bodies, assembled in nine sections around a tentacled core. When attacked Legion will drop zombie like bodies from its carapace, and these will march back and forth across the room, getting in Alucard's way. Occasionally the boss will also let out a scream and a rain of bodies will fall, filling up a good portion of the floor below.
At least one section of the shell has to be eliminated to Alucard can deal damage to the core below, but be wary: when a section of the core is exposed it will then be able to fire off lasers at the hero. If all the pieces of the core are removed before the demon is killed, then it will not only be able to fire off lasers but will also fire shots that home in on the hero. It's better to eliminated to core, then, before all of the shell is removed this time around.
When Legion next appeared it was in a very different form: a lesser enemy floating around in the high sections of Carmilla's Castle in Castlevania: Circle of the Moon. This weird beast, resembling three heads massed together with tentacles emerging from its mouths and joints, vaguely resembled the core of the bigger Granfaloon seen previously. The only attack this Legion has is to touch the player, inflicting the curses status effect if it does so, but otherwise it cannot do anything to the player, acting as more of an obstacle than an outright menace.
Legion returns, in proper form, with a vengeance in the next game in the series, Castlevania: Harmony of Dissonance. Here we actually have to deal with two Legions, Saint Legion and Corpse Legion. Saint Legion appears in the Sky Walkway and, functionally, it's not that different than the original Legion (although it is smaller, likely due to the limits of the game hardware at the time. A four-sectioned beast, with a three-tentacle core, Saint Legion flutters back and forth in the room via its distinctive wings. It won't drop bodies when attacked, but it will summon little versions of itself that will slowly fly at the hero, but these can be killed with a single hit. Destroying all four sections will expose the core, and then all three tentacles will appear and will fire lasers randomly while flying back and forth at the hero. Touching Saint Legion, at any point, will confer the curse status effect on the hero.
The second Legion, Corpse Legion, appears later in the Skeleton Cave. This Legion has an outer form that is actually indestructible; you can't break portions off the core but, instead, have to attack the exterior over and over to get the shell to opening up down the middle. This will, eventually, expose a skeleton at the lower-section of the core that this skeleton is the actual weak point. Attack it repeatedly and keep the damage going or the core will close up and the process will have to be repeated. When attacked, the shell will drop skulls to the ground, while attacking the core will cause maggots to drop and crawl across the screen. Like with Saint Legion, touching Corpse Legion will confer the curse status on the hero.
The Legion in Castlevania: Aria of Sorrow is a more traditional form of the monster. Appearing in a large room in the Underground cemetery, the demon will slowly follow the hero around the room, seeking to touch the hero and deal massive damage. Attacking the shell (of which there are four parts) will, of course, eventually break it away, but you can deal damage to the core even with the shell in tact just be doing enough long-range damage across the carapace. However, if you kill Legion before all four sections of the shell are removed you will not get his soul; you must destroy the shell fully before killing the core if you want the soul. Once the core is exposed, it will, of course, fire lasers at the hero. If all the sections are removed, the core will begin randomly bouncing, at high speeds, around the room before starting its slow pursuit again. This happens in phases, with plenty of lasers flying, so be wary of the core if you want the soul.
Legion then arrived again underneath Garibaldi Temple in Castlevania: Curse of Darkness, and this time around Legion is both very powerful but also entirely optional. The mass of Legion floats in the middle of a tall tower seemingly made of pulsing, throbbing meat, and the only way to reach the body is to climb a spiral staircase of bones around the outside of the room. The shell will not attack the hero in any way, but the corpses that drop off will shamble towards the hero and explode, dealing a not insignificant about of damage. Clearing the shell isn't difficult, although a bit time consuming, but once the shell is removed the core will then be exposed, that the core is very, very powerful indeed.
The core of Legion, called Nuculais (likely a mistranslation of the "hepburn" for "nucleus" as the being was born in the center of Legion) resembles a giant being a light. Nuculais will perform a fair number of melee attacks, swinging at the hero with his arms if they get close (once or several times in combination), turning his arms into blades and dashing at the hero, or growing a long blade and slashing in an arc at the hero. He can also dissolve into the ground and move around, effectively teleporting anywhere he chooses. He can also float up off the ground, then charge up a beam that he will fire at the ground wherever the player is standing, forcing them to run around until the beam ends.
Once he's below half health he'll fall to the ground, shoving a hand into the floor so he can pull out a body to eat. This won't heal him but will, instead, allow him to grow much larger. In this giant, hulking form (that fills most of the height of the tower, he'll slowly march around, causing shock waves with every step. He'll also jump and pound the ground, causing even bigger shock waves Air-combos are the game here to deal damage, just be careful of where you land afterwards. After enough time has passed he'll shrink back down to normal size and can then be pounded into oblivion for whatever health he has left. This can be a long, protracted, and sometime hectic fight across its three phases, but the Nuculais form is much easier to handle than he might at first seem. Just be careful, and get damage in while you can, and you'll have him down.
The Legion that appears in Castlevania: Portrait of Ruin is derived from his appearance in Symphony of the Night, both in its graphics and attacks. You will have to destroy all the portions of the shell, fully exposing the core, but you don't have to do this bit-by-bit -- simply layering on damage on one section, and the core beneath, will eventually cause all the sections to fall away. Once a section of the core is exposed it will, of course, fire off lasers regularly at the hero. Once all the sections of the core are exposed it will also regularly start slowly spinning, chasing the hero with beams of light in a spiral pattern, and will continue to do this regularly until the core is fully defeated.
This version of Legion, from Portrait of Ruin, is also the version that appears in Castlevania: Harmony of Despair. The only real difference this time around is that the spiral pattern of the long laser blasts can start unpredictably, keeping the heroes on their toes as they handle this fight.
Finally, Legion appeared, in its standard form, as one of the bosses in Castlevania: Grimoire of Souls, with the player forced to once again attack the shell before taking out the core, avoiding its usual laser attacks. Although the game used polygonal graphics instead of sprites, the basic look and feel of Legion retained its traditional styling.
Other Appearances of Legion
Like a few other characters from the series, Legion makes an appearance in the (largely not-canon) novel Akumajo Dorakyura: Kabuchi no Tsuisokyoku (translated as either Demon Castle Dracula: Reminiscence of the Divine Abyss or Castlevania: Ricordanza of the Gods Abyss of which a fan-translation is available). In the novel, Legion is brought back by the Puppet Master so it can create a facsimile of the castle of Graham Jones and continue the work of Jones's Dracula-worshiping cult. That beast was eventually destroyed by Michelle DaNasty and Julius Belmont.
While Koji "IGA" Igarashi did oversee the creation of the novel, the book itself was never released outside of Japan and, this, the book can't really be considered canon anywhere outside of Japan. If Konami ever released the book in other territories (an unlikely event at this point as the novel was published all the way back in 2008) its place in the series could be reconsidered. For nor, though, it is apocryphal at best (especially when the canceled game Grimoire of Souls would seem to take place in the same era that would have been set aside for this novel, contradicting its events.
Castlevania Netflix History:
A version of Legion appeared near the end of the third season of the Castlevania Netflix Anime. In that season, villain Isaac was making his way across Europe to get to his one-time partner, now sworn enemy, Hector, but this forced him to cross the lands of a powerful, aged sorcerer, The Magician. The sorcerer had, months before, cleared out the land of all its inhabitants, using his magic to take over their minds and bodies, using them to build him a massive tower.
When Isaac made his way into the remains of their town, on a quest to the tower to steal the sorcerer's magic, the wizard commanded all his bodies together, forming them into a spherical mass, the Legion. Isaac, and his summoned demons, had to fight the mass while dealing with the bodies that would drop off. In the end, though, it wasn't the core that Isaac had to defeat, but the sorcerer himself, to destroy the Legion. Once the sorcerer was dead, the sphere fell apart as all the bodies it contained lost their magic and dropped dead.