Rienfleche is a Rank-S Bow exclusive to Fire Emblem: Rekka no Ken. It is obtained in the final chapter and is dropped by the morph of Uhai. It is regarded as the best Bow in the game and is the 4th most powerful weapon in the game overall.
It is a highly augmented form, in strength and accuracy, of both the Silver Bow and the Brave Bow, with a better might per weight ratio than either weapon.
It appears as though the name of this weapon, comprised apparently of the French rien, "nothing", and fleche, "arrow", is a misrepresentation of Royalefleche, meaning "royal arrow", which would have been more in keeping with the names of the Regal Blade, Basilikos and Rex Hasta. Interestingly enough, it could also have rather easily meant to be called Roifleche, meaning "king arrow", or Reinefleche, meaning "queen arrow".
Rienfleche's in game description states that it can strike consecutively, in the same way as a Brave weapon, but this is false. If the effect were to actually work, it would probably be the best S-ranked in the game, and would also surpass Radiant Dawn's Double Bow if it didn't have 1-2 range.