From WoWWiki

Will of the Forsaken
Will of the Forsaken Instant Cast Self range Provides immunity to Charm, Fear and Sleep while active. May also be used while already afflicted by Charm, Fear or Sleep. Lasts 5 sec.
Usable By
Class	All
Race	Forsaken
Properties
Type	Defensive
School	Physical
Cooldown	2 minutes
Related Buff
Will of the Forsaken Immune to Charm, Fear, and Sleep. Duration: 5 seconds

Will of the Forsaken is a Forsaken racial trait (Undead) which provides immunity to Charm, Fear, and Sleep for 5 seconds. This may also be used while already affected by Charm, Fear, or Sleep to cancel that effect.

Until Patch 1.6, Will of the Forsaken was a 20 second duration but was nerfed and decreased to 5 seconds. Cannibalize was tweaked to offset this change.

The ability is similar to that of the Faction Insignia trinkets.

See Also: Cannibalize, Underwater Breathing, Shadow Resistance