The Hero of The Forest of Doom was a "skilful warrior" and sword for hire. At the beginning of the book, he or she was half a day's walk north of Yaztromo's Tower, having been roaming the "northern borderlands" of his or her kingdom (which may refer either to Chalice or Salamonis or an otherwise unknown kingdom in roughly the same area, or even be an historical term from the days when this whole region was part of the kingdom of Old Allansia) for ten days.
The hero discovered from a dying Dwarf called Bigleg that Gillibran's magical warhammer had been lost within Darkwood Forest and subsequently set out to retrieve it. After returning the weapon to Stonebridge, the hero was rewarded with a golden winged helmet worth several hundred Gold Pieces and a silver box containing dozens of jewels and gems.
It is possible that the hero also took part in other books, either before or afterwards. However, since he or she had never heard of Yaztromo or Gillibran before the start of the book, any previous adventures must have occurred in a different region, ruling out, for example, Caverns of the Snow Witch.