You get this after compleating Act 1(duh!)
The last thing you do in Act 1 is defeat the Berserker.
I strongly recomend playing on Hardcore if you aren't compleatly hopeless in this game since the achievements stack. Also the Berserker only gives you an achievement if you beat it on Hardcore so getting this far on Hardcore nets you 60g(!) instead of just 10g for casual.