I read the comments here, which is the quest you need from Khadgar to start the Nightfallen
and a lot of the comments are talking about the quest not being available, admittedly from back in BfA, and things to try.
One of them does mention doing things in the Class Hall.
I know you said that Wowhead was no help, but I usually find something in the comments.
If you’re still stuck after trying those, it could have something to do with Dalaran phasing related to the starting quests of TWW. Please post again, and maybe someone can help.
Another option might be to Party Sync with a lower level character, who might getyou to a different phase. I’m going out now but will be back in an hour, and I can try that with you.
i might have fixed it though. It seems as though i needed to have all the other reps on friendly before Khadgar gave me a quest. I have a nightborn quest called Trail of Echoes. Not sure if its the one i need but ill update here if its not.