Gladius is one of Roses weapons, which can be found under the sand pits of the Death Frontier.
Of Rose's arsenal, the Gladius is second only to the Dragon Buster and has a chance to inflict death (Can't Combat) with a given probability.
Map[]

Click on the map for a larger image.
Gladius is on the surface at the spot marked "G" on the map.
Note letter "E" is not an item, but a healing pool similar to the one found in Dragon's Nest. There are also two (2) letter "H' shown, but only the one next to letter "I" leads to an item.