You become as hard to stop as death itself. The duration of all Stun and Fear effects used against you is reduced by 10/20%, and your mounted speed is increased by 10/20%. This does not stack with other movement speed increasing effects.
Reference to the last of the Four Horsemen of the Apocalypse in the Book of Revelation, chapter 6, verse 8: "And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him."
Patch 4.0.1 (2010-10-12): Now a Tier 2 Frost talent (was a Tier 4 Unholy talent), and reduces the duration of movement-slowing effects instead of Fear and Stun. Icon updated.