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. When Scanlan attacked and hit, he was able to make a second attack at disadvantage.
Notable Uses Edit
When Scanlan scried on Kaylie for the first time, Matthew Mercer said that Scanlan had given Kaylie the "Aberration's Bane longsword" (capitalized in episode transcript). It's unclear whether the Singing Dawnblade is based on a Bane weapon (which does extra damage to one type of creature, much like dragon slayer weapons) carried over from Pathfinder, or Mercer thought Scanlan had given her a different weapon.