The Trial of Champions

If you're alliance looking for where to pick up these quests to work on Housing, its from Exarch Yrel at The Lion's Den (Tanaan)

This does still leave one spot on the Council empty.

Rangari Prime - Leader of the Rangari - Naielle
High Vindicator - Leader of the Vindicators - Akama
Sha'tari Proconsul - Leader of Shattrath City - Othaar, traitor
Speaker for the Dead - Leader of the Auchenai - Maladaar
Chief Artificer - Leader of the Artificiers - Hataaru, dead

We don't know which one Yrel took up, but I assume Sha'tari Proconsul since I can't see her being an engineer. Hataaru's spot is still open though.

I posted this under the "Garrison Campaign" but I think it might be helpful here as well.

I was going crazy trying to figure out how to find these quests! Hopefully this helps someone else out there!

The Garrison Campaign is made up of 12 weekly quest lines. Each one is started in your garrison by a quest giver.

Bloodmaul Compound is started by (A)Bodrick Grey/(H)Rokhan
Broken Precipice is started by Nixxie (A/H)
Darktide Roost is started by Bodrick Grey/Rokhan
Exarch’s Call is started by Deedree *This is the questline for The Trial of Champions** (I believe it's Alliance only). Ours is the Fury (horde side) is started by Thrall, but don't know if it's useful for decor.
The Fall Of Shattrath is started by Khadgar’s Servant(A/H)
Heart of the Breakers is started by Morthis Whisperwing/Choluna
Iron Siegeworks is started by Barros Alexston/Rokhan
Ring of Blood is started by Overly Gaudy Note (A/H) (this is hanging on the wall inside the Town Hall)
The Sargerei is started by Khadgar’s Servant (A/H)
Warlords Council is started by Bodrick Grey/Rokhan
A Strike at the Heart is started by Baros Alexston/Warmaster Zog
Primal Fury is started by Bodrick Grey/Rokhan

If you are like me and could not find ANY quests in your garrisons, **you were most likely in the middle of one of these questlines sometime in the past and abandoned the quest**. You will need to figure out where you left off so that you can continue.

Although it is time consuming, you will need to check the quest IDs of each step of each quest line. The master list of all quests can be found here: Garrison Campaign Quests.

If you’ve never done this before, here is how to do it: From the master list above, click on the “>Complete quest chains (click to view)” tab under each yellow heading. Then select the first entry under the Alliance tab if you’re Alliance or the Horde tab if you’re Horde. You will then see all of the required quests in the series in a list on the right hand side. You need to click on each of these to get the Wowhead page for each one to load. Then, on the left hand side, if you scroll down, under “Gains” and before “Guides” you will see a chat command that you can click to copy. Paste that into your chat box in game and you will see a “true” if you’ve completed the quest, “false” if you haven’t (alternatively, you can make a macro and change just the 5 digit quest id for each, which is the number in the parentheses). Once you find a quest chain that starts with a “true”, then skip to the last quest in the chain. If that is also true, then you have completed that quest chain with that toon. If the first quest in the chain is true, and the last quest is "false", then you have found the chain you’re stuck on! Go back to the top of the list and check the quests in order one by one until you find the first one that says false, and that’s the step you’re on!

To make things just a bit more challenging, I was on a step that required a disguise to see the quest giver! All I can say is that if you find what step you're on, but can’t see the “!”, look up the specific quest in Wowhead and see what you need to do. Luckily, I clicked on the quest giver, even without the “!” and he told me to go get a disguise and then come back and talk to him again. So frustrating!

Edit: Each of your alts has the possibility of a different quest being available. I usually cycle through 7-8 alts and see 3-4 different quests available. The achievement is account-wide, so it makes sense to check all of your alts. It says in the beginning of the Garrison Campaign post, "There are 12 quest chains total and you can complete one roughly every week. The order is mostly randomized except for a handful which come at the end." The ones for the decor are likely the ones at the end (since I haven't seen them yet on 8 alts), so do the quests every week on as many toons as you can to get there!

Edit 2: I ended up getting the two decor campaign chapters before I had seen all of the available quests. I had only completed 6/12 when I got the Fall of Shattrath and 7/12 when Deedree finally had the Exarch's Call quest. So they are not the last two you get. Good luck everyone!