The Singing Dawnblade is a sword that Scanlan Shorthalt used before acquiring the legendary Mythcarver. Scanlan asked Kerrek to modify the scabbard to create a tiny storage slot, into which he inserted a dab of Scanlan's Poo of Scrying before giving the sword to his daughter, Kaylie.
Because Scanlan so rarely makes weapon attacks, the Singing Dawnblade saw very little use even before it was replaced by Mythcarver. However, we can glean the following from Scanlan's two attacks with the sword in "The Throne Room" (1x07):
The sword attack is 1d20 + STR + Proficiency. The damage die is at least a d8 (consistent with a longsword or rapier). When Scanlan attacked and hit, he was able to make a second attack at disadvantage.