It may be involved in an upcoming update, or its development and release may have been cancelled entirely.
Praetorian armour is a set of unreleased Magic armour, comprised of Zarosian artefacts, which would have been uncovered at the Kharid-et Dig Site as part of the Archaeology skill. It is not currently known if the armour will ever be released.
At present, only some pieces of the Praetorian armour set can be found in-game, though they may currently not be worn. Worn models have been discovered within the cache and the set has been confirmed to exist by Mod Rowley. This is similar to other partially, or completely, unreleased equipment sets from the Archaeology skill, such as the Legionary, Venator, Pontifex, Icyene and Priest of Ikov armours.
Whilst the armour set can not currently be obtained in its entirety, a blue translucent version of the set and weapon can be seen at Kharid-et in the form of Ghost Praetorians. Players can currently obtain the hood, robe top, and staff; but not the robe legs, boots, or gloves. Arguably, the Inquisitor staff is also part of this set, and can be obtained in-game.
