The legend of the blade Yusaris predates Andraste. The sword that Dane found in the dragon's treasure hoard, which he used to slay both Fenshal and the werewolf, was passed on to his son Hafter. Dane may have been fiction, but Hafter was fact. In 1:40 Divine, he led the Alamarri tribes against darkspawn that flooded into the Ferelden valley from the dwarven lands. After years of ruling the valley in peace, it is said that Hafter left Ferelden, sailing into the unknown east of the Amaranthine Ocean with the blade still in hand, never to be seen again. —From Codex entry: Yusaris: The Dragonslayer
Yusaris was possibly sent to the Circle Tower for authentication by the author of A Very Chewed and Moist Book. Considering the rambling of the rest of the book, it's possible this particular codex entry was actually intended for a different codex entry and was left as an artifact during programming.