These are deities which originate from the Zoroastrianism religion which was founded by the Prophet Zoroaster in ancient Iran around 1500 BC.
The most important texts of the zorastrianism religion are those of the Avesta, of which a significant portion has been lost, and mostly only the liturgies of which have survived. The lost portions are known of only through references and brief quotations in the later works, primarily from the 9th to 11th centuries. In some form, it served as the national or state religion of a significant portion of the pre-Islamic Iranian people for many centuries.
The Avesta says that: At the end of the “third time” (or perhaps, third Age), there will be a great battle between the forces of good and those of evil in which the good will triumph. Two copies were created, one which was put in the Persian house of archives, and the other put in the Imperial treasury, but both were supposedly destroyed by Alexander the Great’s conquest of Persia. Remaining sections were taken by the Greeks.
In texts evolved from the Avesta, deities known as Asuras and Deavas are described. The Asuras were originally just, good, and virtuous, but their nature had gradually changed with new political alignments and alliances. As well as with changes in moral conceptions and ritual, these gods changed sides. On the other hand, in the original Avesta, they are described as wise and good and it is in fact the Deava which were the enemy.
What is clear, is that as documented by the ancient Zoroastrian religion, the warring between Asura and Deava may have been more of a philosophical difference rather than as physical enemies. So perhaps neither were in the right or wrong, they were simply opposing factions in the god wars. Furthermore, depending on which text at what time is referenced, both Asuras and Deavas have been villainized. What is also agreed upon is that both were powerful and mighty.
We know far more about the Asura than the Deava. For instance, the Asuri is the feminine form of these gods. Whereas the Asur refers to the masculine which were keepers of Fire, and known as Asha, and acted as agents of truth and justice. In reflection of their deity, such gods bore the title “Asha” — a term likely devolved from earlier etymologies as the words (Asha and Asari) sound phonetically similar to titles of channelers from the Fourth Age.