Vengeful Nether Drake

Source
The Vengeful Nether Drake was the reward for players in the top 0.5% of arena teams per battlegroup at the end of Season 3.

Mount Journal
The most aggressive nether drakes were awarded those heroes who showed significant prowess and commitment in the gladiatorial arena.

Achievements
Qualifiers for this mount should also get the Feat of Strength Vengeful Nether Drake (achievement).