i guess it is like Same like mimirons head. Most ppl won't see it drop ever
The drop chance has been lowered I think
As of the last patch, 4.0.3a, Invincible's Reins has been lowered to a 1% drop rate, as has Mimiron's Head. Oddly enough, blizzard has not updated the item data on their website and the item still reflects a 100% drop rate from 25m Lich King Heroic: Link.