To get the Enchanted Gladius you must collect the three effigies:

Ruby: Chapter 4 - In the room before up the the ladder before the rubble which you must climb over.
Sapphire: Chapter 8 - In the room before the one with the Forgotten Corridor Key, use a 5 point Reveal Invisible to show the effigy.
Emerald: Chapter 11 - Protected by a damage field next to the worms.

After getting all effigies, go to the room with the plastic explosives and put the red effigy on the blue wall, green on red and blue on green opening a secret passage with a permantely enchanted gladius.