Affixes your weapon with a rune that causes 2% extra weapon damage as Frost damage and increases enemies' vulnerability to your Frost attacks. Modifying your rune weapon requires a Rune Forge in Ebon Hold.
Rune of Razorice is a Death knightfrost rune that can be made through [Runeforging]. Dual-wielding frost DKs will use this on one of their weapons (main hand seems to be the consensus with [Rune of the Fallen Crusader] on the other) to boost their frost damage on single target fights.
Patch changes
Patch 3.3.3 (2010-03-23): Now stacks 5 stacks of 2% Frost Vulnerability instead of 10 stacks of 1% Frost Vulnerability. Proc chance changed to 100%.