That's a pretty tragic story. I'll only go over the basics. Kabr, Praedyth, Osiris, Phanin(speeding, sorry) and two unmade guardians went into the vault after Osiris's urging to understand the vex and deny their power. The only known for sure survivor is Osiris. Phanin survived, made it out and back to the vanguard but what he saw in there as well as Kane's death drove him crazy to the point of death. Praedyth is alive but isn't at the same time, forever stuck in conflicting timelines of the vex. Kabr is interesting. He was the leader and long after the rest of his team was either dead or escaped he was alone. Phanin did see him die so it's confusing but we know he was alone because he tells us in his grimriore and other sources. He went as far as he could until he couldn't go any more and sacrificed himself to create the aegis. Whenever you use the aegis in vog, you are in a sense using Kabr cause he poured his life and light into it. The other two are unnamed, we don't know if they're still alive, died in the vault, escaped but then died or what. That's why they went in and how each member fared.

Osiris sent him in an effort to understand the vex. Unfortunately all but one member of the fireteam died and he went so crazy he made a talking gun.