This time I beat them without so much preparation as a level 21 (get 22 while fighting them) hard cleric.
I'm impressed, you managed to beat the strongest faction of the exiled kingdoms all at once alone. That's the only challenge of the entire game I haven't done yet. I have beaten every other foe, gained every achievement and conquered even ironman as a mage, but against the full forsaken force I have always bitten the dust so far. What you did is a great achievement and shouldn't be taken lightly
Still, this was my hardest battle, and I get nothing to remember it, not even little achievement
While an achievement might be a bit too much to ask since only a very select few of players could even hope to defeat this challenge, it would indeed be nice if you got some item or something as a reward for your troubles. And proof that you did the near impossible.
Although, you do get the reward of completing the only ending where you can beat up the magister for all the trouble he has caused you. In all other endings he either leave peacefully or flees away so you finally have the satisfaction of giving him a well deserved punch in the face for kidnapping you and your sister.