User blog comment:Vitalius/The return of Oración Seis and Crime Sorcière/@comment-24066070-20131023130053/@comment-24066070-20131023153714

Strengthening their forces is done by gathering useful members of the smaller dark guilds. These useful members, or at least some of them, then get "amplified" by Kyouka. I think that over the course of 7 years the smaller guilds under OS and GH have either disbanded or merged into those under Tartarus, therefore making it easier for Tartarus to find the remaining dark mages.

About CS, I think it pretty much is their life goal as they believe they can only "atone" for their sins by eradicating all the remaining dark guilds, demons, Zeref himself, ... Besides they can't really do anything else at the moment, given the fact they are both wanted criminals. They can just hope their criminal records will be cleared by the new MC who takes their good intentions into consideration.

I know there are quite some "I think"'s in my reasoning, but that's the impression I get from them.