Around 100 B.C., the prophet Zoroaster reformulated the original polytheism of Persian religion into the  first monotheism. Although little is known about the old Persian polytheism, apparently some of the old gods were demoted and retained as angels in Zoroaster's synthesis.

The struggle between good and evil occupies center stage in the Zoroastrian worldview. The god of light and the upper world, Ohrmazd or Ahura Mazda (Wise Lord), and his angels were locked in a cosmic struggle with the god of darkness and the lower world, Angra Mainyu or Ahriman (Evil Spirit) and his demons. Every human being is urged to side with Ahura Mazda and his angels.

Chief among the Zoroastian angels are the holy immortals (amesha spentas),  the highest beings next to God in the Zoroastrian scheme of celestial inhabitants. These beings are named after qualities valued by Zoroastrians, and are usually numbered as six archangels:

Vohu Manah (Good Thought or Good Sense)

Armaiti (Piety or Harmony)

Ameretat (Immortality)

Asha (Righteousness or Truth)

Haurvatat (Prosperity or Salvation)

Kshathra (Power or Rulership)

This list is sometimes supplimented by a seventh, Saraosha (Obedience).

Zoroaster originally regarded these beings as aspects of Ahura Mazda himself. Corresponding to these archangels of light are agents of the evil Angra Mainyu, such as Druj (the Lie), who was directly opposed to Asha.

The holy immortals played an important role in the  religious experience of the founder. At the age of thirty, Zoroaster had a vision of Vohu Manah, who appeared to the prophet in a form of nine times larger than an ordinary person. After questioning Zoroaster, Vohu Manah led his spirit into the heaven of Ahura Mazda, who was holding court with his angels. God then instructed th prophet in the principles of the true religion. Over a course of the following eight years, Zoroaster had a visionary experience with each of the archangels - experiences that allowed him to fill out the initial revelation into a complete religious system.

Resource: Angels A-Z,  Dorothy Oliver and James R. Lewis, pages 188-189