The common noun, which also appears in the Younger Avesta (e.g. Yasna 61.5), is also used as a generic to denote religious leaders and another common noun airyaman "member of community" is an epithet of these saoshyants. In contrast, the standing epithet of the saviour figure(s) is 'astvat-әrәta' "embodying righteousness," which has arta/asha "Truth" as an element of the name. These saviours are those who follow Ahura Mazda's teaching "with acts inspired by asha" (Yasna 48.12).
'Saoshyant' appears as a proper name in the Younger Avesta, explicitly so in Yasht 13.129 where it is used in the singular and where 'Astvat-ereta' is also invoked as an alternate name of 'Saoshyant'. The singular also appears in Yasna 59.1 where Verethragna is said to be Saoshyant's weapon in overcoming resistance. A plural form appears for instance in Yasht 17.1 where Ashi -- the divinity of "recompense" -- is described to give the Saoshyants the power of "making wonderful" (frasho.kereti).
Zoroastrian tradition (already envisions three future saviours, one for the end of each 1,000-year period that comprise the last 3,000 years of the world. All three will be born of maidens, conceived while their mothers bathed in a lake that miraculously preserved the seed of the prophet Zoroaster himself. The first will be named Hushedar, the second Hushedarmah, and the third will be Saoshyant, who will lead humanity in the final battle against falsehood.
The story of the Saoshyant's conception and early life are described in Denkard 7.10.15ff as follows: Thirty years before the decisive final battle, a maiden named Eredat-fedhri ("Victorious Helper") and whose nickname is "Body-maker" will enter a lake (in Yasht 19.92, this is "Lake Kansava"). Sitting in the water, the girl, who has "not associated with men" will receive "victorious knowledge." Her son, when born, will not know nourishment from his mother, his body will be sun-like, and the "royal glory" of Khwarenah will be with him. Then, for the next 57 years he will subsist on only vegetables (17 years), then only water (30 years) and for then for the final 10 years only on "spiritual food."
The events of the final renovation are described in the Bundahishn (30.1ff): In the final battle with evil, the yazatas Airyaman and Atar will "melt the metal in the hills and mountains, and it will be upon the earth like a river" (Bundahishn 34.18), but the righteous (ashavan) will not be harmed.
Eventually, Ahura Mazda will triumph, and his agent Saoshyant will resurrect the dead, whose bodies will be restored to eternal perfection, and whose souls will be cleansed and reunited with God. Time will then end, and truth/righteousness (asha) and immortality will thereafter be everlasting.