Very long story short, the Pagans - a group of raiders working under King Krezzman pretty much kidnapped a bunch of mercs and forced them to work for them or end up dead. They wanted to know if Donna Lazari was killing their dudes and sent them to find out.
The group generally got distracted or got dead, with Dallen seeming to be the sole exception. He has met up with a 'vaultie' who found her companions murdered by said Pagans. Prior to this the Desperados casino found itself the scene of a shootout between Redding and what was left of the fiends. There was much fire.
Now it would seem the pagans have tired of waiting and are seizing New Reno aggressively. A few other newer characters have cropped up and there are one or two I haven't mentioned thus far, but you get the picture in a nutshell. Any more specific questions then just ask :)
The Pagans don't work for Krezzman.
The Pagans are the gang employing the player characters.