Blade Ward

Permanently enchants a weapon to sometimes grant Blade Warding when striking an enemy. Blade Warding increases your parry rating by 100 and inflicts 286 to 315 damage on your next parry. Lasts 10 sec. This enchantment requires the wielder is at least level 75.

Source
Taught by.

Mechanics
The enchant procs about 5% of the time and lasts for 10 seconds. If you parry during this time, the enchant will cause damage. If the enchant procs again during the 10 seconds, it will refresh and stack, up to 5 times. Stacking up 5 would be exceedingly rare given the chance to proc. If you do not parry in the 10 second time frame, no matter how many stacks you have accumulated, the effect dispels.

Here is some data collected from actual combat logs. Applications is the number of time the effect appeared. Lands are only counted when effect hit the target. All other applications were unused or stacked with additional procs. † Actual percentage of hits parried, not parry derived from gear

This enchant would particularly useful for tanks who parry frequently, have a fast weapon, and tank multiple mobs simultaneously. The spell damage done is affected by spell power. This effect does have the ability to crit.