Barshanöm is the most important Zoroastrian purification rite. Barshanöm consists of full 9 nights of solitude, ritual purification and reflection. The 9 nights of seclusion rite is primary to gain insight and wisdom. The purification rite is invoked against the forces of death, decay and evil and is seen as awakening the inner eye.
Every Zoroastrian priest has to undergo barshanöm before being initiated herbád/ervad and again before solemnizing the rituals.
Also, every member of the Zoroastrian community is required to undergo the full 9 nights rite at least once in his or her lifetime.
According to the Persian Rivayats, barshanöm is part of the preparation required of coverts to the Zoroastrian religion (Persian Rivayats, ed. Unvala, I, p. 282; tr. Dhabhar, p. 276).
The purification rite is touched upon briefly in the Yasht/hymn dedicated to haúr-vatát (healing, cure, every wisdom and wholeness formula) and described in detail in Vendidad 8.37-72, more briefly in Vendidad 9.1-37.
The full 9 nights of purification, seclusion and reflection of the Zoroastrians, reminds one of Óðinn’s discovery of the runes after full 9 nights.
Óðinn watched the Norns from his seat in Asgard and envied their powers and their wisdom. And he bent his will toward the task of coming to know the runes.
Since the runes’ native home was in the Well of Urd (Compare with the Avestan ardá/arthá) with the Norns of faith, and since the runes did not reveal themselves to any but those who prove themselves worthy of such insights, Óðinn hung himself from a branch of Yggdrasil (tree of life,) pierced himself with his spear, and peered downward into the shadowy waters below. Óðinn stared downward and called to the runes for no less than nine days and nights, teetering on the realm that separates the living from the dead.
At the end of the ninth night, Óðinn at last perceived shapes in the depths: the runes! The runes had accepted his sacrifice and shown themselves to Óðinn, revealing to Óðinn not only their forms, but also the secrets that lie within them. Equipped with the knowledge of how to wield the runes, Óðinn became the mightiest and most accomplished of beings in all the worlds.
After the full 9 nights, Óðinn’s gift/offering/sacrifice to himself (gefinn Óðinn) secured him from the Well of Wyrd 18 (twice 9) charms or runes.
I shall add that the poetic gathas or the most sacred verses of the Zoroastrians consist of 17 songs and 1 chapter of 7 charms, or 18 in total. The 18 gathas/songs (Lithuanian giedoti “to sing”) are world-shaping mind/consciousness formulas or secret wisdom (avestá from the root vid.)