My theory is Kabr is Atheon and I know this sounds crazy but hear me out. In the Grimoire cards it says the Vault could have created Atheon, not the other way around. Also Kabr never speaks of Atheon but he does speak of the Templar, how he made it past the Templar, not dying to the Gorgons, and such but never mentions Atheon. He also says at the end of the Grimoire card for The Aegis that if he ever speaks again, he is not Kabr. And think about it. Croata's sword defeats Croata while The Aegis ( The Shield) basically defeats Atheon. He also said that he made his armor out of dead Vex parts but the Vex got inside his blood and mind which fits Atheon since Atheon wears Vex armor and is controlled by the Vex. Even by his name though, Kabr the Legionless, it suggests he was an explorer and who was to say that Atheon wasn't even around when the Vault of Glass was forming if you remember that part from earlier and thought Kabr would be perfect to be the ruling, so to speak, God of that place. Plus they took his ghost so who knows what they could do to him without the light to guide him. This might not be true but I think it makes lots of sense so please spread this around if you also think it makes sense.