Even though I think the Divine should have the killing blow on Damian I also think are dragon knight should be the main playable charactor in D3. The reason being the Divine has already shown what he's capable of he had his campain that turned him into a demi god. He's had a lot of time to show the world what he can do now that are dragon knight has found his/her meaning & come to terms with & understand what it means to be a dragon knight. It just would'nt be fair to not give are knight a chance to shine & prove themselfs like the Divine has. Let the Divine Correct his mistake but let the dragon knight shine as well, that would make for an epic tale.
Have the dragon knight as your main playable charactor while at the same time have the Divines side of the story run paralel with yours. Maybe for the Finally hold off Damians forces while the Divine has his showdown with Damian, or after you've done everything you can to ensure success take controle of the Divine for the final showdown.