Feast of Blades
Following the Iron Cage campaign, Rogal Dorn was ready to break up the VII Legion into Chapters to comply with the Second Founding. However, he wanted the Fists successors to remember that they were still his gene-sons, and to maintain good relations with the Chapters. To achieve this, Dorn decreed that every century, the Chapters bearing Dorn's gene-seed would come together in the Feast of Blades, a tournament where champions from each Chapter engage in sword duels. At the end of the tournament, the winning (sometimes surviving) swordsman is awarded the Sword of Sebastus, or Dornsblade, the power sword Rogal Dorn used after shattering his original chainsword for failing to save the Emperor during the Siege of Terra. Any relics once wielded by a Primarch is usually worth more than a solar system to the Imperium, so as you can understand, turnout is high.
The novel Legion of the Damned gives a look at what the Feast of Blades is like. Competitors wear carapace armour (for some the first time they worn such since they were Scouts), and given envenomed blades which will slow down a Space Marine when cut. The arena the duelists fight in is constantly shifting, to represent the chaotic environment of the Iron Cage.
The current holders of the blade are the Excoriators who, notably, did not win by being the fastest, strongest, or best with a blade but by being prepared to take the most damage and outlast their opponents.