The Ahamkara are disciples of the Worm, not the Worm themselves. Crota was tricked by Oryx sibling into cutting time/space inside Oryx Throne World. The Vex emerged from that rift. Crotas punishment was to be tossed into the Vex gate to fight his way to glory or die forgotten. Crota became a legend, but didn't fight vex, as his sisters told him to stay away, essentially. Crotas greatest achievement was conquering the moon and killing the guardians. For the effort, the Shrine of Oryx was built inside his Temple. (Tossed that one in)