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Becoming a Shadoweave Tailor
To specialize in Shadoweave tailoring, a character must complete Neutral [70] Becoming a Shadoweave Tailor, given by Andrion Darkspinner <Shadoweave Specialist> in Shattrath City. To obtain this quest you need to have 350 proficiency in Tailoring, and be ready to travel to Shadowmoon Valley. You don't need to be level 70.
The quest requires you to travel to the Altar of Shadows and attune a crystal to it, then travel back to Shattrath City to complete the quest.
The Altar of Shadows is located inside the Netherwing Pass, a valley in the cluster of mountains in the southeast corner of Shadowmoon Valley. The pass begins in the middle range of the mountains (coords 62,63) and the Altar is on the edge of the cliff, directly south (coords 58,71).
You don't need any mats to complete the quest, and it won't start the cooldown on Shadoweave crafting. Immediately after completing the quest you may travel back to Shadowmoon Valley to craft your fist two pieces of Shadowcloth.
It doesn't state a minimal level restriction.
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SHADOWEAVE TAILORING