The Blade of Marmora is a group of Galra resistance fighters that has existed for many centuries.
The Blade of Marmora has a hidden main base, shown in the episode 'The Blade of Marmora', Ulaz's outpost, as well as others. The Blade of Marmora has spies working on the inside of Zarkon's command hierarchy, who work with Voltron to take down the Empire piece by piece.
The Blade of Marmora oppose Zarkon's rule over the universe after discovering that the Emperor had more secretive intentions than to sustain peace across the universe. They are allies to the Voltron Force, and even risk some of their own for the sake of intelligence, per their mantra, "Knowledge or death." (A variation of the Galra Empire's "Victory or Death.")
Members (also referred to as "blades") are put through a strenuous amount of trials in order to determine if they are worthy to join the ultimate fight against the Galra Empire. Keith, who was obsessed with the origins of his blade, underwent the trials with great persistence. It was only when his lion attacked that he chose to relinquish it, which in turn, activated the blade. The only way that a blade can be activated is through an example of true self-discipline and heritage of Galra blood. This also revealed that Keith is descended from a Blade, and is therefore a legacy member of the organization.
They each wield a blade made of a rare metal known as luxite. The blades possess the ability to transform from their dormant state, which is typically a small knife inscribed with their insignia, into a larger sword, comparable to an Egyptian khopesh in size. The design of both the dormant and awakened states varies from member to member.