Just got it myself.
[12:18:03] [Azgul] has earned the achievement [Deathcharger's Reins]!
[12:18:03] You receive loot: [Deathcharger's Reins].
Done 50 runs post 3.0 on my human rogue, done about 50 as well prior to that. Then add the 600+ I've done on my undead rogue in TBC.