The infamous apparatus of execution that was the central point of France's Reign of Terror, a period during the French revolution shortly after the the First French Republic was established. An apparatus that Robespierre used to execute countless counter-revolutionaries, only to be used to behead him later on in an ironic twist of fate; officially ending the French's Reign of Terror. At some point in history, the Guilottine had blessed the birth of a particular girl, only to behead her as soon as she had stepped upon the ladder of adulthood; thus becoming the sole object within all of creation capable of housing her soul.
The history contained within such a device is obviously very deep and profound, it's without a doubt that the power this blade of decapitation would carry is equally great. When the Relic isn't armed as a weapon (stage of Assiah), it's capable of unleashing an invisible, flying "blades" in all directions up to approximately 20 meters from Ren's body. While it might not be enough to breach the defense of an Apostle, it's more than capable of completely cutting up the body of an adult human into several pieces.
Ren is able to step further by manifesting his Relic as a weapon; in which case his right arm would turn jet-black, with a massive scythe-like blade protruding out from his elbow. At this point, Ren would lose the capability to unleash invisible "blades" like he could when the blade wasn't manifested. In turn, his physical prowess would be heightened and his blade would perform its original purpose, an act of decapitation, in the highest efficiency possible. Due to the Curse of Guillotine that Marie's soul possess, this Relic's aptitude for killing is dangerously high, matching those of Machina's fist which can end anything in a single strike. No matter how though they might be, even if they're an incorporeal apparition; this blade wouldn't make a single exception. Even if one happened to be immortal; the guillotine shall lay them to rest. Those whose neck have been touched by this guillotine, will surely meet their end.