Ok, here’s the breakdown.
Twilight Destiny: You arrive in Candlekeep and spend a few days there researching. You encounter an old sage named Azagalas who knows a bit about the Wyrmskull Throne. The Wyrmskull Throne was the seat of the King of Shanatar, the greatest dwarven kingdom in the history of Faerun. It has been lost for centuries and most regard it as legend and nothing more. The throne is rumored to have had all sorts of powers. Of course, which ones are just stories and what is true is hard to discern.
Azagalas is willing to help Dargroth find this throne and fulfill his destiny. He also knows other sages and scholars who can help Rhyse find his tower, Cerangil learn more about the Feywild, etc. He says that he is a council member of a large adventuring company called the Champions of Faerun. He would be glad to accept you as members so you can help each other.
Shadow Legion: Azagalas is the patron that the harpers spoke of. Speaking with him, you find out that he knows a lot about Tiamat and believes that the last time Tiamat was banished from the face of Faerun was with the Wyrmskull Throne. Through the various connections CoF has, he can also help with other things such as destroying the Soul Keeper, battling the forces of Shar, etc. He also offers you membership into the Champions of Faerun so you may help each other in your various goals.
I know this is the definition of railroading but it is needed for this whole thing to work. I apologize.
Anyway, that’s the story, post in the comments with any questions you have.