Mazdean (Persian) Demonology
In many ways one of the most remarkable systems of demonic belief is that presented in the Avesta, the sacred book of the Mazdean religion of Zoroaster. In this ancient religion, the war between light and darkness, good and evil comes into great prominence. Against the good God, Ahura Mazda, with his hierarchy of holy spirits, there is arrayed the dark kingdom of demons, or daevas (yes, the word is very similar to the Indian devas - see below), under Anro Mainyus or else Ahriman, the Evil Spirit, the Demon of Demons (Daevanam Daeva), who is ever warring against Ahura Mazda and his faithful servants such as Zoroaster.
The term Daeva is an example of the semantic transformation from "benevolent" to "malignant" which is seen in the case of the Greek word daimon. The original meaning of the word daeva was "shining one", and it comes from the primitive Aryan root div, which is also the root of the Greek Dias (Zeus, God of thunder and lightning) and the Greek daimon, as well as the Latin deus. But while these terms, much like the Sanskrit deva, retain their good meaning §, daeva has come to mean "an evil spirit". It is of great significance that, while the word in its original sense was synonymous with Lucifer (the light-bringer, in Greek Eosphoros, he who brings forth the dawn), it has now come to mean much the same as devil. It is also significant to observe the acoustic similarity between daeva, the modern Persian term dev, and the word devil. If one asks a member of the Christian church to explain the apparent similarity, both semantic and in sound, of those three terms they will say that the word devil derives from diabolus (diavolos - diaballein) and there is really no connection. I believe that there is an apparent connection, both due to the fact that Christianity historically adopted and transfigured many elements of the terminology of the lands it first emerged in, and because the root of the term diabolos (diavolos in Greek, even today) itself is also the Aryan root div. Moreover, if we base another argument on the anthropological claim of the common Indo-European ancestry of all the peoples of the Mediterranean basin, we can as well safely presume that even the Aryan root div emerges directly from the Sanskrit root deva.
Although there are marked differences between the demons of the Avesta and the demons in Scripture and Christian theology, the essential struggle between good and evil is still the same in both cases. The representation of the holiness and fidelity of Zoroaster when he is assailed by the temptations and persecutions of Anro Mainyus and his demons may well recall the trials of saints under the assaults of Satan or suggest some analogy with the temptation of Christ in the wilderness (40 days, if you don't recall. Basically, I would like to point out, as another piece of evidence supporting the anthropological claim I mentioned earlier, that also Buddha and earlier Brahmin figures from Indian scriptures went through this 40-day trial in the desert. There is an almost alchemical significance in this depiction of the Holy Man, which Christianity has also adopted and now claims as its own - for some reason).
The important role played by the demons in the Mazdean system may be observed in the Vendidad, which is the largest and most complete part of the Avesta, so much so that when the sacred book is written or printed without the commentaries it is generally known as Vendidad Sade which means something that is "given against the demons" - vidaevodata, i.e. contra daimones datus or antidaemoniacus.
§ In Greek daimon meant spirit, and not demon. The bad spirit is called kakodaimon, and the good spirit is called eudaimon, hence the term for the state of good spirit prevailing in men, called eudaimonia.