I made a druid in shadowlands and wanted to get this flight form so I went back to legion and did this. The documentation for this is scattered so it's difficult to find all of the information you need. So for the sake of others who might be doing it, I'm going to document as much as I can based on what all I can remember.
You have to unlock the broken shore, which involves completing the Assault on Broken Shore quest.
You have to do your class order hall quests, from Rise, Champions upto and including A Hero's Weapon
After you finish A Hero's Weapon, go back to the broken shore and get Champions of Legionfall. Once you get that you can unlock Talon Terror, and you need to complete it up to and including Champion: Thisalee Crow.
After these are complete, you don't even have to turn in Champions of Legionfall, but you need to return to broken shore and you will receive this quest.