Overview
Fists of Fury: Champion of Champions is the final fistfighting quest in the base game of The Witcher 3: Wild Hunt. It unlocks once Geralt has become champion in all three regions: Velen, Novigrad and Skellige. After the last title, a man invites him to the arena in Hov on Spikeroog.
The fight
The bookie outside the arena will only open it once Master of the Arena is complete. Then he reveals the reigning champion: a rock troll. Food and potions are allowed. You only get one try, so save first. Losing means the Brawl Master achievement is gone for good.
Win and Geralt is proclaimed Champion of Champions, and the bookie promises to spread the news to the four corners of the world. Talk to the troll afterwards to learn his name.