It took me about 6 years or so, but I finally got Attumen to hand over his mount!
I had wanted this mount very badly all throughout TBC when I had been running Karazhan with my guild. Never once did I see it drop.
Later on in Wrath I began to solo Attumen on occasion, still hoping to add this mount to my collection, but no luck. In Cataclysm I tried here and there with no success, then I just kind of forgot about it.
About 6 months ago I decided to give Attumen weekly beatings until he handed Midnight over to me. I’d say I hit Kara probably 3 out of 4 weeks for about 5-6 months. In that time I vendored a boatload of Worgen Claw Necklaces, but still no mount.