Avonal's origin
P.224 - §6 Every time an original and absolute concept of being formulated by the Eternal Son unites with a new and divine ideal of loving service conceived by the Infinite Spirit, a new and original Son of God, a Paradise Magisterial Son, is produced. These Sons constitute the order of Avonals in contradistinction to the order of Michael, the Creator Sons. Though not creators in the personal sense, they are closely associated with the Michaels in all their work. The Avonals are planetary ministers and judges, the magistrates of the time-space realms--of all races, to all worlds, and in all universes.
Avonal's incarnation
P.225 - §6 3. Bestowal Missions. The Avonal Sons do all, at least once, bestow themselves upon some mortal race on some evolutionary world. Judicial visits are numerous, magisterial missions may be plural, but on each planet there appears but one bestowal Son. Bestowal Avonals are born of woman as Michael of Nebadon was incarnated on Urantia.