Final answer:
The Immortals were an elite group of the Persian standing army renowned for their fighting skills and their sustained number of 10,000 soldiers.
Step-by-step explanation:
The Immortals were an elite group of the Persian standing army, not a group of gods worshipped by the Persians or Assyrians nor were they soldiers who died in battle. These soldiers formed a prestigious corps within the ancient Persian military structure and were known for their fierce skills and the policy of immediately replacing any fallen soldier to maintain their constant number of exactly 10,000, hence the name 'Immortals'. This enduring legacy is a testament to the highly organized and powerful military forces established by the Persian Empire, which also included other combat units like charioteers and cavalry.