By being 'allies' with Kogrim, during 'The Sacred Ashes'
I suggest you save the game before walking thru the firewall. defile the ashes, and talk to Kolgrim to earn your reward (reaver class), then you reload that save and dont defile it. class unlocked stays on profile and is permanent. but if you dont reload, Lelianna will attack you if she's there, or leave your party next time you're in camp. + you get huge penalties with lotsa party members, like wynne and alistar.