I am a new returning player - the last I played was Beta, I believe. Summerset advertising brought me back - and I purchased all of the DLC I missed. I'm only stating this, because other people may have a similar experience.
I started my new game in Summerset and completed all of the DLC quests. During the Summerset questline,
Darien decides to sacrifice himself to power a sword.
Upon completing the Summerset main questline, it was suggested I do the original main quest line. Upon reaching Coldharbor,
as Ebonheart Pact, I assemble a team to confront Molag Bal and prevent the planes from melding. Darien was part of this team and pretended nothing happened.
So I am assuming this is a bug. There is a huge hole in the story between the two.