Fanaticism is a retribution paladin Talent.
Increases the critical strike chance of all Judgements capable of a critical hit by 3/6/9/12/15% and reduces the threat caused by all actions by 6/12/18/24/30% except when under the effects of [Righteous Fury].
Notes
Again, it's a "must have" for retadins as it increases critical hit and increases the chance to have the proc of Vengeance_%28paladin_talent%29. With a good critical strike chance, you can reach 50% chance to crit with judgement with [Sanctified Seals] and Fanaticism.
In patch 2.3 it gained the threat reduction component.