Legion is a biblical demon, a massive collection of demonic entities housed in one entity. Wikipedia would be your friend also, if you wish to look into the matter further.
Legion, (named 'Granfaloon' in Symphony of the night), is one of the boss enemies, taking the form of a giant sphere of rotting bodies inhabited by an multi-limb tentacle monster. Its a relatively easy enemy if you already know Soul steal and Bat charge or you have already picked up the chrysaegram sword.
The Name Legion is a biblical reference ref. the Demoniac Jesus heals when he casts the multitude of demons into the swine. The name 'Granfaloon' is a Kurt Vonegut (spelling?) reference and he has said it means 'a proud but meaningless group of people'.
Legion sheds waves of living corpses and, as the fight progresses, fires large lasers. It does not move but its lasers can do a good bit of damage and the bodies it sheds, if you let their number get to large, it can kill you fairly easily.
Legion appears in a few other castlevania games but is almost always a free-floating spherical mass of rotting corpses.
Is there any reason this is tagged castlevania? I mean even previous comments implied that Legion is not exclusive to the Castlevania series; not to mention I've never encountered a version of Legion that looks like this in the games that I've played.