what are a few sources from which we get to know about him?
The Abydos King List, the Saqqara Tablet and on the Turin canon.
Unas
He occupies the 33rd entry of the Abydos King List, which was written during the reign of Seti I (1290–1279 BC). Unas' name is also present on the Saqqara Tablet (32nd entry) and on the Turin canon
who was unas? He was a pharaoh, the ninth and last ruler of the Fifth Dynasty of Egypt during the Old Kingdom.
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Unas [SEP] Historical sources
Unas is well attested by historical sources with three ancient Egyptian king lists dating to the New Kingdom period mentioning him. He occupies the 33rd entry of the Abydos King List, which was written during the reign of Seti I (1290–1279 BC). Unas' name is also present on the Saqqara Tablet (32nd entry) and on the Turin canon (third column, 25th row), both of which were written during the reign of Ramses II (1279–1213 BC). The Turin canon further credits Unas with 30 years of reign. These sources all place Unas as the ninth and final ruler of the Fifth Dynasty, succeeding Djedkare Isesi and preceding Teti on the throne.