Netherlord's Chaotic Wrathsteed

Having just spent best part of half an hour searching Dreadscar Rift and Wowhead and reading half a dozen guides on how to complete this quest, nobody seems to have given the information I needed to actually start the quest line. In case anyone else has handed in the last quest of the Breaching the Tomb, mounted up and flown back to Dalaran (or hearthed) and now have no idea how to get on the first rung that all the guides tell you to, don't! Stay there turn around and go to 44,63. Speak to Elithys Firestorm who will offer a quest "Bloodbringer's Missive."

Now go back to the class hall, hand in and start the questline.

If this helps even one other person, its been worth it :)

Here are all of the colorations of the mount:

The original: Netherlord's Chaotic Wrathsteed. You get this mount by doing your class mount quest.

Red/Orange/Black recolor: [item=143637]. You can buy this mount for 1000x[currency=1220] from [npc=101097] in your order hall, but you need to have Power Ascended on the character that you're buying this on - you need to have 52 or more traits on your artifact weapon.

Dark Purple recolor: [item=142233]. This version of the mount drops from Lord Hel'nurath on Broken Shore after you get your Netherlord's Chaotic Wrathsteed. [item=142233] has 100% drop chance. The mount will still drop even if you don't see the skull icon on the map for Lord Hel'nurath.

Edited for clarity.

The below is accurate for anyone who is completing this on an alt. You do not need to do any zones/reputations/quests to get this to work. This presumes that:

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• You have completed Legion questlines/required content on a main previously.
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• You have taken & skipped the questline to get to Dalaran from Orgrimmar/Stormwind.
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• You got into your Class Hall.
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Since I did not find updated information for completing this on an alt easily, here is what I found, accurate as of the moment the changes were made to open all appearances to alts if a main completed the requirements in Legion (so 2023 and onwards, for sure):

1. 
   
2. Complete Uniting The Isles @ Khadgar in Dalaran - again, it is not required to have any reputation or do any pre-requisites. Go to Violet Citadel, take and turn in the quest instantly. You will need this to get WQs to do for your Class Hall campaign. You can do this at any point once you enter Dalaran.
3. 
   
4. Finish A Glorious Campaign achievement (your Class Hall campaign) - it takes roughly a couple of days, depending on your effort, but does have some timed missions you will have to do. Note that you can focus on the CLASS Campaign rather than the ARTIFACT Campaign - check your quest log to see where the quests fall if you are unsure. You will need to do the initial weapon quest and maybe a couple to set up your hall, but you don't have to get any reputation or go to any Legion zones to quest. I'd recommend doing all the artifacts for every spec just to have them and not get confused. Completing this should award you the Class Title at the end. If you did not receive it - you have not finished! Make sure you also complete the Heroic Weapons quest that pops up after the last quest of the campaign.
5. 
   
6. Start Legionfall Campaign @ Krasus' Landing in Dalaran. You can skip ahead and not do the scenario.
7. 
   
8. Grab the Champions of Legionfall quest from Maiev - if it is not available, you need to finish your CLASS campaign. You don't need to do any other quests in the zone.
9. 
   
10. Head to your Class Order Hall.
11. 
    
12. Take and complete the questline for the new champion offered to you.
13. 
    
14. Once completed, you might see a quest on the map in your hall, but you can't take it/find it - go to Legionfall in Broken Shore again, submit Maiev's quest, and you should be able to start your mount quest right there. The mount quest is picked up in Legionfall, but it can take a second to load in. If you are not seeing it, try relogging.
15. 
    
16. Do the quest to obtain your mount. This will unlock all mount options that exist. Be slow with DPS as this has bugged out for me multiple times when I went too fast during the event. If that happens, you can jump off and start again.
17. 
    

Congrats, you are done! Some rewards may require extra effort, but all are available to get after these steps.

Some of warlock's unique Class Hall rewards are:
Netherlord Title
Chaotic Wrathsteed (Mount)
Brimstone Wrathsteed (Mount)
Accursed Wrathsteed (Mount)
Hateful Eye (Pet) - also available for demon hunters

You can refer to other rewards/classes in this great guide:
Class-Specific Toys, Mounts and More!

P.S. I would rank the warlock questline for the mount one of the worst. Requires quite a few things to collect and do, and you do need to get a 100% drop from a rare mob for one of the recolors, unlike other halls where you can just buy it.