While I was working on raising my reputation with Hydraxian Waterlords by soloing Molten Core I got two halves of the legendary sword as a windfall. It also required 10xEnchanted Elementium Bar which on a good day runs about 1,000 gold each on AH, but as soon as I got my pieces jumped to 5,000 gold apiece. I just went out and farmed some of materials for the bars and bought the rest of the mats on the Auction House and one of my guild-mates smelted it for me, so my overall cost was about 4,000 total for the whole package.
The quest is confusing, you return mats and halves to the NPC and nothing happens. Good thing someone was floating on a magic carpet right above me, I figured his target was a boss right behind me, so I turned and killed it, returned the drop to the NPC, got the sword back and received a guild achievement. Amazingly, level 25 guild with 400+ members and I was the first to get it. Now, I still didn't get the personal achievement until I actually equipped the Thunderfury, Blessed Blade of the Windseeker, which I was able to do, even given the fact hunters are not allowed to equip melee weapons anymore. Or are we?