I checked the first 10 pages and also did a search for "tabard" with no results matching this, if this is a repost just link it and don't rep me.
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Step 1. Take your Green (Purple) Trophy Tabard of the Illidari
Step 2. Destroy it.
Step 3. Go to Ironforge Welcome Building
Step 4. Talk to Lyesa Steelbrow
Step 5. Select the tabard you DIDNT just destroy (If you had green, choose purple)
Step 6. You now have +2 tabards towards your achievement.
NOTE THIS ONLY WORKS IN IF NOT IN SW
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Midsummer's tabards DO NOT WORK LIKE THIS, DON'T THROW THEM AWAY!
Real Edit: Yes, I've been playing too long:
- Ashbringer
- Sulfurus
- Scepter of hte Shifting Sands
- GM 2H
- Quel Serrar
Also, various crap from my long travels. No, it's NOT a private server.