You have accomplished the truly impossible. The brood mother of the Black Dragonflight lies dead at your feet. Seize her head and deliver it to Zardeth of the Black Claw.
By Nozdormu's Teeth!
What? How did you get this? Isn't she... Never mind the details, I suppose. You'll be wanting one of these baubles as recompense, no doubt.
The reason that this quest doesn't turn in at King Varian Wrynn (or Bolvar Fordragon in earlier versions of WoW) and does not give a city-wide buff as it did in the level 60 version is because in Patch 3.2.2 Onyxia is officially dead according to Warcraft lore. Though players still have the ability to kill Onyxia as a boss, in Warcraft lore she was killed by a party led by Varian Wrynn, Broll Bearmantle, Valeera Sanguinar, and Thargas Anvilmar. This would also explain why the quest name has changed to "A Most Puzzling Circumstance" and why Zardeth shows such surprise upon seeing the head of Onyxia.
Blizzard put in a number of rather subtle "jokes" into the quest such as stating in the quest text that "You have accomplished the truly impossible," referencing of course that although Onyxia is still a level 80 raid boss, she is officially dead according to lore.
Patch 3.2.2 (2009-09-22): Added.