Recently I went back to get my DK class mount from Legion. I had finished the order hall campaign and figured it couldn’t be too much work to get the mount. At one time you had to finish the Legionfall campaign, but it seems that’s no longer the case. I doubt it took me longer than an hour or two to get the mount. Spoilers for the quest series follow.
The quests involve finding the remains for a powerful red dragon and raising it in undeath. One quest has you enter the Ruby Sanctum. Originally, there was a feat of strength you could earn for slaying all the red dragons in the sanctum, but Blizzard removed it because they decided it wasn’t fair to give a reward like that for something you could miss. Since it was removed by the time I tried to do the quest, I took the opposite approach; I got in and out without killing anything.
Here’s a video I edited together of the lore parts of the quest series. Starting at 9:26 you can see the three forms of the mount. I mostly play unholy, but I think it’s the worst variation. I like the frost one!