So right now I'm long done with the main questline as a Vampire Lord, I'm reading that you get that shout after defeating Durnehviir after leaving with Sarena's mother's elder scroll. I defeated him the first time before receiving the scroll, then a second time when he's waiting for you outside of the prison after getting the scroll, and I immediately left to the Soul Cairn exit. Was I supposed to stay or something? I passed by the area again recently to get Arvak's skull but the dragon was no where to be found. Am I screwed?
EDIT: Also, was I supposed to get the "Legend" achievement from that too? The one that says to defeat a Legendary Dragon. I didn't get the achievement.