The Iron Sword is the initial weapon Luka wields before it is replaced by the Angel Halo. Luka uses the Iron Sword against Slime Girl, Slug Girl, Mandragora, Granberia's first encounter, the final encounter with Lazarus, and Alice in both fights.
This sword is notable in the fact that, as a regular mundane weapon without special abilities, it takes lives, as opposed to sealing monsters like the Angel Halo does, and thus plays an important role when Luka attempts to kill Lazarus in their final encounter, and with Alice in their second battle.