Completed Legion and got my mount, and this is how I got my mount on my alts:

I am creating this to inform anyone else who will inevitably run into the same issue. I completed the Legion campaign and got my Breaching the Tomb achievement on my Paladin before War Within started. Well, since the start of War Within, I have my Warlock as my main. I finally decided to get the class mount on the Warlock, and with Warbands feature, I didn't have to do the achievement again. However, it's confusing. You must do the intro quest (you can skip if you have before) first. After you've unlocked world quests and the questgivers on Deliverance Point, you can now get the Champions of the Legionfall quest. You must complete this quest, and for my Warlock that was recruiting Kanrethad, for other classes, just go to your campaign map and find out what champion(s) you haven't unlocked yet and go unlock them to complete the quest. Once this is done, your quest for your mount will appear. (ofc you have to get your artifact weapon.) This took me a couple weeks and chats with a GM. They linked to me some comment threads they found and I was able to take that info and piece everything and figure it out. Good luck, and if anyone in the future has questions, feel free to ask and I will see what I can do to help.