Mubarizun was the special unit of the Rashidun army

Rashidun army

The Rashidun Caliphate Army or Rashidun army was the primary military body of the Rashidun Caliphate's armed forces during the Muslim conquests of the 7th century, serving alongside the Rashidun Navy...

. It was composed of the elite warriors, who were the champion swordsmen, lancer

Lancer

A lancer was a type of cavalryman who fought with a lance. Lances were used in mounted warfare by the Assyrians as early as and subsequently by Greek, Persian, Gallic, Han-Chinese, nomadic and Roman horsemen...

s and archers

Archery

Archery is the art, practice, or skill of propelling arrows with the use of a bow, from Latin arcus. Archery has historically been used for hunting and combat; in modern times, however, its main use is that of a recreational activity...

. In that era, battles usually were preluded by duels between the champion warriors of the opposing armies. The Mubarizun (literally: duellers) duels would have a large psychological impact on the other combatants, even more so when the duel was between high ranking officers. Thus the purpose of this unit was to slay as many commanders of the opposing army, in a duel, as possible, for the purpose of demoralizing the enemy soldiers.
Some of the most famous Champions were:

• Khalid ibn Walid
• Zirrar ibn Azwar
• Qa'qa ibn Amr
• Asim ibn Amr
• Abdulrehman ibn Abu Bakr (Son of Caliph Abu Bakr
  Abu Bakr
  Abu Bakr was a senior companion and the father-in-law of the Islamic prophet Muhammad. He ruled over the Rashidun Caliphate from 632-634 CE when he became the first Muslim Caliph following Muhammad's death...
  
  )