These Relic Weapons are only available to Red Mages and Paladins. As such, only these jobs may use this Weapon Skill. Blue Mages may use this Weapon Skill under specific conditions when equipped with the Corbenic Sword.
"Knights of the Round" was also a Summon (red) Materia in Final Fantasy VII; when used, it would summon thirteen knights one at a time (presumably King Arthur and his knights), who would each attack all enemies for high damage, which could cumulatively exceed the 9,999 HP damage limit for attacks. The Materia could only be obtained by completing the "Chocobo Breeding" side quest or by destroying the Ruby Weapon.