The Dragonguard invasion of Tamriel was fueled not only for their oath to exterminate the remaining dragons, but to search for the fabled Dragonborn. They arrived at the northernmost point of Tamriel, Skyrim, and marched south defeating scores of both Dragons and Nords alike until they were stopped by Reman Cyrodiil at Pale Pass.
Reman was the first person who united the human lands of Cyrodiil and, in doing so, defeated the Akaviri invaders. The remaining Dragonguard, upon hearing the voice of Reman, knelt and swore their lives to him. They had found the Dragonborn they were searching for.