ALright and Ill now admit I thought it was after my mistake... but that does not explain how he did the curse and I think if a mortal can curse an entire army for an eternity then a red eye can manifest on top of a tower
here's my second less magical answers
He filled the souls of the warriors with such guilt that they did not feel worthy of death and stayed to the land.... but why curse them only if you win.... It would make more sense to curse them if you lose (indeed curse them either way) but if you lost it may have been the lack of warriors you had and their army was the amount need to overpower the dark lord...