Flask of Rejuvenation is a relic piece obtained by defeating Ascended Immortal, the 1st Boss in the Canyon of Immortality. Alternatively, you can purchase it from the Adventure Store for 575 Smith Tokens once you have 2-Star Mastery in the Canyon of Immortality.
The other pieces you need to create Sirisi are the 3 Rank-A Mithril Swords; Spellvoid, Excaliber, and Isiris. As you can see in the picture below, it will tell you how many correct swords you have. The below picture has 1 with a gold border, so it still needs 2 Rank-A Swords. Hovering your mouse over the grey swords will tell you which Rank-B swords it requires.
Successfully fusing the 4 components together over 36 hours will result in your sword.