These elves were originally of a Highborne clan that renamed themselves appropriately to Dawntreader to follow their sister clan's change to Sunstrider. They were among the earliest settlers to the Eastern Kingdoms, and married into the nephews and grand-children of the great Dath'Remar Sunstrider to keep their blood pure.
The Dawntreader line went on to serve High King Anastarian diligently as Councillors and Advisors during his long reign within the Sunfury Spire, until his death at the hands of Arthas Menethil.
From Dawntreader to Dawnsorrow
After the Sacking of Quel'Thalas and the Great Genocide of The Elves, what was left of the Dawntreader line fled from the bloodied city to parts unknown. Changing their name from Dawntreader to Dawnsorrow served two purposes; To dissuade any looking to snuff out the last of the Thalassian Royalty, and as an homage to all that was lost. Directly after changing their last name, so too did the words. For the exact reason of the last name, as well as to inspire Dawnsorrow Knights.