The Mercenary appears in Heroes III: The Restoration of Erathia. He is the player avatar and unseen protagonist of the Spoils of War campaign, a roving mercenary and general whose skills are in demand to turn the tide of the border war between Krewlod, Tatalia and Erathia.
The Mercenary initially fights for both Krewlod and Tatalia, participating in a gold rush to finance Krewlod's offense and assisting King Tralossk's warriors in holding and breaching the Erathian border and obtaining several resource mines. In the ensuing direct skirmish between the forces of Duke Boragus and King Tralossk, he chooses either side and leads them to an initial victory. The choice is left to the player, and is canonically ambiguous.
However, the Mercenary's achievement is later nullified when king Gavin Magnus sends Bracadan reinforcements to Erathia in order to quell the conflict between Tatalia and Krewlod while retaking the lost land.
He apparently employed independent advisors affiliated only with his cause, as one addresses him "Milord" in the opening of Borderlands. His true name and identity are entirely unknown.