Mubarizun

The Mubarizun (, "duelists", or "champions") formed a special unit of the Rashidun army during the Muslim conquests of the 7th century. The Mubarizun were a recognized part of the Muslim army with the purpose of engaging enemy champions in single combat.
In Arab, Byzantine, and Sassanian warfare, battles usually began with duels between the champion warriors of the opposing armies. The Mubarizun were the bravest men of the islamic armies, and a whole myth surrounds their name. They would stand alone in front of the entire army, engaging in mortal combat for the pride of their nation and companions.
The Muslim army would typically begin battle with its soldiers first equipping their armor, assembling their units to their positions and lastly dispatching the Mubarizun. Mubarizun fighters were instructed to refrain from pursuing any defeated enemy champions more than two-thirds of the way to the enemy lines to avoid the risk of being cut off. After the conclusion of the dueling phase, the army would launch its general advance.