Allegedly, some people in Stormwind can make oils that will help in the fight against the Legion, and they need Nether Residue to do it. This quest is assigned by Emisarry Mordin who paths near the flight master.
For this quest, all you need to do is be a skinner, herbalist and/or miner, and go out and do some gathering. Any Outlands herb (not Golden Sansam or Dreamfoil), ore node, or (I assume) skinnable mob has a chance to drop Nether Residue, of which you need eight for the quest. The drop rate on herbs felt like around 30%, possibly higher on higher-level herbs.
So it's a fairly easy quest to do, and as a bonus, you get some herbs/ore/leather. Go on back to Shattrath to turn it in, and be rewarded with 16g, 250 Shattered Sun rep, and