Kabr the Legionless was one of the most powerful Guardians in history. It's any speculation as to how he opened the vault. As somebody mentioned earlier, it may have been possible that the rest of his fireteam had been erased from existence by The Templar, the Oracles, and the Gorgons. If they no longer existed, than he wouldn't even remember them. Thats why, although he says that nobody was with him, there's something missing from his memory, and all he recalls are blank figures, running, shooting, and helping him. So, although he can no longer recall the names or faces of his comrades, pieces of his memory show him that he was, indeed, never alone. When they encountered the Templar the darkness was too strong, and Kabr and his presumed fireteam were outmatched by the Templar and its Oracles. One by one, the Oracles and the Templar erased his team. This explains how he knows what the Oracles and the Templar are able to do, without having had to experience it himself. Kabr, and possibly one other, only survived because, in a last-ditch attempt, their Ghosts used all of their light to cleanse the darkness from their masters' bodies before it could negate them from existence. This sacrifice was what killed his Ghost, which is why he says that they have taken his Ghost from him. After Kabr and his remaining comrade escaped from the Templar's domain, they encountered the Gorgons. Knowing that the forces inside the Vault were beyond reality, they attempted to use stealth to pass by them, but were caught by one of them. They responded with overwhelming force and destroyed the Gorgon, but at the cost of his final teammate. After destroying the Gorgon, he became insanely curious and obsessed with the reality-bending powers of these Vex, and took its body with him, to fashion it into armor. He believed that, by wearing these Vex's armor, he would be unkillable, and might even gain their power. Determined to see the axis mind of these powerful Vex, he made his way to the Glass Throne, and fought the forces inside to awaken it. However, when he became marked by the void, that is when he started to feel the Darkness consume him. He opened the second gate, killed the gatekeeper, then noticed the massive change to the area, and surmised that the two portals were of the past and the future. As his vision was nearly gone, he saw the massive glasslike Vex known as Atheon appear before him. He then knew that time was being warped in this place because of the Darkness. He wanted to know if there was any weakness or counter to this temporal force, a way to fight back against the darkness that was controlling time: Light. Light, fused into the nearly indestructible armor of the Gorgons, would be impossible for the Darkness to consume. He took the remaining armor from the dead Gorgon, fashioned it into a shield, then took the light from his dying body and infused it into the shield, then, as Atheon attempted to send him into the Timestream to be lost within time, Kabr sent the shield into the Timestream instead, along with his final words and curse. Unlike the rest of this presumed fireteam, Kabr was not erased by the Vault. Instead, he simply fought until the darkness began to consume him. That's why, although nobody remembers Kabr's team, they remember Kabr, and thus remember him as "The Legionless."