The origin of Legion's design is a biblical account of a large number of demons who possessed a man. The New Testament outlines an encounter where Jesus healed a man from Gadara possessed by demons while traveling, known as "the exorcism of the Gerasene demoniac". Thus, from the concept of being "composed of many" came the design of the shell composed of a large number of human bodies.
The Symphony of the Night boss, Granfaloon, is named after a concept from a 1963 novel called Cat's Cradle by Kurt Vonnegut, Jr. In the novel, a "Granfaloon" is a satirical reference to a group of high-minded people who share a similar belief. The group is described by Vonnegut as "a proud and meaningless association of human beings". The Granfaloon in Castlevania would be a literal representation of this idea.
A seething lump of flesh made from innumerable human sacrifices. Crushes foes with its massive weight. The general design is a floating sphere composed of numerous featureless humanoid bodies that acts as a shell concealing a center core of varying identity. Generally speaking, the shell has to be broken apart by repeated strikes before the core is exposed, and only by damaging the core can the boss be defeated.
Legion is found as the boss of the Catacombs. The shell has nine parts (arranged like spaces on a tic-tac-toe board), although just by destroying the center section is sufficient to expose the core. The shell "attacks" by dropping bodies which fall to the ground and then walk aimlessly like zombies. If Alucard is closest to a non-destroyed portion of the shell, Legion will occasionally let out a growl and then a scream, followed by a shower of bodies raining upon him. If the portion of the shell Alucard is closest to is destroyed, then the newly exposed core tentacle (out of eight) will start firing off powerful laser beams instead.
Legion can be defeated without first destroying all nine shell parts (eight (inter)cardinal directions and one directly covering the core), but if all shell parts are destroyed, then inbetween single laser shots, Legion's tentacles will also fire off sets of homing laser shots that are very difficult to avoid.
The Legion found in Circle of the Moon is very different from the rest of his other incarnations. For starters, it is a lesser enemy, not a boss. Instead of being a composed of many full corpses, it is smaller and is only composed of an amalgamation of heads. It's still relatively large, however, has a rather deceptive high mobility, and will inflict Curse status on contact.
Legion (saint) is found in the Sky Walkway in Castle B and is similar to the Symphony Legion, but only has four parts, and destroying any one of them will expose the core. It apparently floats in mid-air using a pair of angelic wings (only a graphical feature, not gameplay-relevant). Also, Legion (saint) does not drop bodies, but will only occasionally produce smaller versions of itself which fly over to the player for physical damage and inflicting Curse status. Destroying the shell reveals a small core with three tentacles which can periodically shoot dashed laser beams.
Legion (corpse) is found in the Skeleton Cave (a catacombs-like area) in Castle A. Instead of a shell of bodies, this Legion is protected by a shell of bones, mostly human skulls - a shell that is actually indestructible. However, hitting this shell sufficiently causes it to open and reveal a bobbing-head skeleton at the bottom of it (along with a graphical touch of flies escaping), and this skeleton is its actual weakness. The skeleton must be hit repeatedly quickly enough or the creature will attempt to close itself again. If the shell is to any degree open, it will also drop strange-looking maggots that crawl slowly along the ground toward the player. Also, notably, even well into its death animation, Legion will still stay onscreen, damaging the player if it touches it during this time (contact confers Curse status, as well as damage to Juste and Maxim; Simon Belmont cannot be cursed).
Similar to its depiction in Symphony of the Night, Legion is an amalgamation of featureless bodies, this time arranged to resemble a fetus during a sonogram, surrounding a tentacled golden cage containing a demonic fetus.
It can be found at the end of the Underground Cemetery, after Soma tracks its shambling husks through the level to a large, square room. Once Soma enters the room, the last of the bodies are sucked into the sphere and Legion awakens.
Legion slowly chases Soma around the rectangular circuit of the room, occasionally shedding husks. It consists of four shells parts and a core. Each of the shells has 5,000 HP, 55 ATK and 0 DEF. While the core can be damaged without destroying any of the shell parts, the boss's soul, Legion Laser, will not drop unless all the shells have been completely destroyed before the boss perishes. As the shells are removed, the tentacled core will begin firing lasers at Soma, and if the shell is entirely removed, the core will gain the ability to infrequently retract the tentacles and rapidly bounce around the circuit.
Once slain, Legion's central cage will shatter and release the demonic fetus, which will either immediately die in a burst of flames if defeated while still having one or multiple shells attached, or float up, open it's eyes, and vanish in a small circle of light if defeated after destroying all the shell parts, yielding its soul.
Legion was brought back along with the Puppet Master by the remnants of Graham's cult during a partial eclipse. He created an imitation of the castle from Graham Jones' time by using bodies of humans, as well as other monsters, and which was disguised by Count Olrox. Everyone killed in the castle was assimilated by it.
Legion is fought in a tall room, seemingly composed of flesh, under Garibaldi Temple. The mass hovers in the middle of the room and is reached by climbing a spiraling walkway protruding from the wall. Legion itself does not attack in any way, but featureless zombies will infinitely rise from the floor and explode if they come into contact with the player (for around 200 damage at Level 50).
When Legion is defeated, it releases a giant translucent white humanoid with an exposed heart known as Nuculais. Nuculais is one of the hardest bosses in the game and has many attacks, including turning into a mobile pool of lava, throwing the aforementioned zombies at the player (which explode on contact), and a powerful charge attack by turning its hands into blades.
Graphically ported from Symphony of the Night, in Portrait of Ruin Legion appears as the boss of the Nation of Fools. Like his counterparts from previous games, he is protected by a core made with the bodies of innumerable men which fall and pace on the ground when the core is attacked. Legion himself is a grotesque flying being with bearing nine tentacle-like limbs.
He cannot be defeated without destroying his outer shell first. Even if the player concentrates their attacks on the core, all sections will eventually fall anyway, revealing the monster himself. In this form, Legion will not only fire the blue lasers seen in other games, but it will also spin slowly and launch fire under pressure from the tentacles.
Legion reappears as a Vital Soul boss. It was left significantly stronger thanks largely to Hermina's meddling by rewriting the text, resulting in the 1797 version of Maria Renard being left in grave danger. After its defeat, Alucard, upon gaining its Vital Soul, ended up encountering Soma Cruz, who apparently succumbed to his true nature as Dracula before Hermina revealed herself.
An entity that strikingly resembles Legion appears in the manga/anime Soul Eater. It's unclear if this is a direct reference to the Castlevania character, as Legion first appeared in 1997 in Castlevania: Symphony of the Night and the Soul Eater manga came out much later, in 2003, and its anime adaptation in 2008.
Legion's core somewhat resembles depictions of the ophanim, a type of angel that is said to hold up God's throne, and among the highest ranked in the angelic hierarchy. They are described as several wheels spinning and rotating within each other, similarly to a gyroscope with rims covered in eyes.