How do you get Legion legendary?

Hello there, I'm curious about something. Could you possibly explain to me the process of acquiring a Legion legendary in the game? I've heard it's quite the challenge, and I'm eager to learn more about the steps involved. Are there any specific quests or dungeons that I need to complete? Do I need to have a certain level of gear or character progression? Any insights you could provide would be greatly appreciated. Thank you in advance!