The Blade of Marmora is a secret organization from Voltron: Legendary Defender opposed to the Galra Empire, though most if not all of it's members are at least part Galra. They are allies of the Defenders of the Universe and known for their attire, which typically consisted of armor or robes with hoods that could produce a fully concealing face mask.
The Blade of Marmora was formed some centuries ago by Galra who had come to see that Zarkon's rule had not brought the stability they had hoped for when he pushed for expanding the Empire. Dedicated to overthrowing Zarkon, the Blade worked both within the Empire and outside of it to sabotage the Galra expansion and aid others involved in the fight. The signature weapon of the warriors of their order was a ceremonial blade decorated with a glowing Galra rune and made out of Luxite, a material from a planet that no longer exists. Once a Blade-as individual members were known-had "awakened" their weapon, they gained the ability to cause it to change size. However, in order to awaken it they had to pass through the Trials of Marmora, a grueling initiation ritual that, according to their leader Kolivan, would lead to either "Knowledge or death."
The first contact the Blade of Marmora had with the Paladins of Voltron was when Ulaz, a Blade who had infiltrated the Galra Empire as a scientist, freed future Black Paladin Shiro. After uploading coordinates to a Blade of Marmora outpost into Shiro's robotic arm, he enabled his escape and planted a bomb to serve as a distraction. Shiro later remembered this after the first battle at Galra Central Command, and once Pidge identified the coordinates they traveled to the base. Ulaz would then board the Castle of Lions, and soon told the Paladins about the Blade of Marmora and provided them with information to get to the headquarters. After the outpost was attacked by the Prorok Robeast, Ulaz sacrificed himself to destroy it, and urged the Paladins to find the Blade of Marmora after they had determined how Zarkon was tracking them.
After succeeding in weakening Zarkon's bond with the Black Lion, Shiro led his fellow paladins to the Blade of Marmora's headquarters, which was situated in orbit of a blue star caught between two black holes. He and Keith then landed on the base and were admitted, only to find that Kolivan, the Blade of Marmora's leader, wasn't as convinced of the worth of an alliance as Ulaz had been. He was especially suspicious when Keith violated their request to come unarmed and was revealed to be carrying a ceremonial blade, which they believed he had stolen. Keith, who had possessed the weapon for his whole life, refused to give it up, and so was subjected to the Trials of Marmora, which he passed when he proved willing to give up his Blade. It's awakening revealed that he was part Galra, and Kolivan and Antok then returned to the Castle of Lions with them in order to plan the final assault on Zarkon.
The Blade of Marmora's urgency was brought on by word from Thace, their spy in Galra Central Command who had previously enabled Voltron's escape, that the Galra Empire leaders were on his trail. As such, they worked with the Defenders and the Olkari to implement a plan that would send Galra Central Command to a distant region of the universe and disable its systems so that they could cripple it. Their plan succeeded, though only with Thace sacrificing himself to reset the system so that they could upload a virus. When Voltron subsequently became involved in a battle with Zarkon's Armor, Allura led Kolivan and Antok to attack Haggar and the Druids, who were attempting to interfere. They succeeded in defeating them, but Antok perished as a result.