The core is usually a round creature with laser-shooting tentacles, but varies slightly in some games. In all Legion's appearances, it is defeated by knocking off chunks of the corpse ball to expose the core, and then finishing it off. Initially, it's main attack is simply dropping bodies at the player which then start walking around like zombies, but as pieces of it fall off the tentacles start shooting lasers, thus making Legion more dangerous as it becomes more vulnerable.
Legion appears in Symphony of the Night (where he was called "Granfaloon" in the first English version), Harmony of Dissonance where he appears twice, first as a "Saint" version that is similar to the original but has angel wings, and later on as a "Corpse" version which is made of bones), Aria of Sorrow, Portrait of Ruin, Curse of Darkness (where he is a secret boss and has a humanoid second form called "Nuculais"), and Harmony of Despair.
Original Series: Secondary