IIRC there was a Vice article following their rise in Syria. I know I linked it at least twice in one of the Syria threads.
All of this is TIFWIW from my memory of 8ish years ago.
Assad was fighting a bunch of people. ISIS was one of them but they werent a threat. The main threat was whatever the Syrian freedom group called itself. (Peoples army of something, made up of all the old veterans Assad fired) They had pushed Assad way back.
Now Assad had a bunch of old Al Queda and other terrorists locked up from an exchange with the US, that's a fun rabbit hole worth reading that you should like. They got out. Again IIRC there was some confusion. There might have been an internal jail break, or an outside strike happened and some prisoners got free. Now part of the confusion is that the jail claims these leaders had already been moved before the incident and were in another facility. But they were actually free.
What made the Peoples Army dangerous, and ISIS not dangerous was leadership. The peoples army was made up of vets. Who were effective leaders. ISIS had little foothold because they had no leadership. Of course some of the leadership that had been locked up were exIraqi military so now ISIS was effectively lead.
Also ISIS was able to change narrative. The Peoples Army was fighting a pure government based rebellion and was able to draw support from a lot of disgruntled people that were all anti Assad and didnt have too many issues with each other because it was focused on other issues. But with ISIS on the rise the religious support was drawn away from the Peoples Army and now Assad gets to claim he is fighting Islamic extremists.