After seeing the ending of Ego Draconis, where it was revealed that the Divine One was in fact still alive, it seemed only too appropriate that the Dragon Knight only take down Ygerna.
Damian's anger is directed towards the Divine, he didn't really care much about the Dragon Knight - that's made most notable in the scene in the Battle Tower - with Zandalor there, he focused on him and ignored the Dragon Knight.
But Ygerna, that is who the Dragon Knight has the grudge against. Ygerna is the one who was in the DK's head all game long, talking to them, guiding them, lying to them. Ygerna is the one who manipulated and betrayed the Dragon Knight, and so it is most fitting that Ygerna is the climactic battle of the Dragon Knight's story.
Yes, Damian is still out there. But the Divine is back as well, and Damian is the Divine's mistake to deal with. It's only fitting for the Divine to play a major part (if not the leading part) in the defeat of Damian.
