Wikia

Will of the Necropolis
Will of the Necropolis (Passive) Passive When a damaging attack brings you below 30% of your maximum health, the cooldown on your Rune Tap ability is refreshed and your next Rune Tap has no cost, and all damage taken is reduced by 25% for 8 sec. This effect cannot occur more than once every 45 seconds.
Usable by
Class	Death knight
Properties
School	Physical
Talents
Specialization	Blood
Level required	70
Related buff
Will of the Necropolis Damage taken reduced by 25%. Duration: 8 seconds
Related buff
Will of the Necropolis Your next Rune Tap has no cost. Duration: 8 seconds

Will of the Necropolis is a passive death knight ability learned at level 70 for death knights with the blood specialization.

Patch changes

Patch 4.0.6 (8-Feb-2011): No longer refunds a Blood Rune when it resets [Rune Tap]'s cooldown. Instead it makes the next Rune Tap free while Will of the Necropolis is active.
Patch 3.3.3 (2010-03-23): There is no longer a cooldown on the frequency at which this talent can be activated. In addition, this ability can now also be triggered by damage which deals less than 5% of your health.
Patch 3.1.0 (14-Apr-2009): This ability now has a 15 second cooldown on how often it can occur. No longer triggers from damage amounts less than 5% of the player’s health.
Patch 3.0.8 (2009-01-20): Now reduces the damage of any attack that takes the DK below 35% health by 5/10/15% instead of boosting armor when wounded. It no longer affects the cooldown of Anti-Magic Shell and retains the current damage reduction bonus.