A Most Puzzling Circumstance (Horde)

Objectives
Take the Head of Onyxia to Arnak Fireblade in the Cleft of Shadow.
 * Head of Onyxia (Provided)

Description
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 Arnak Fireblade.

Progress
What is the meaning of this?

Completion
You've brought me the head of someone who is... No, best not to think about it too hard. You'll be wanting recompense for your heroic deeds, no doubt.