History of Exemplar of Justice

Exemplar of Justice is an Awoken Favor attunement. It improves melee recharge rate for you and your allies, and it is earned by landing final blows with an Ability.

It is unlocked at the War Table in the H.E.L.M., under Upgrades > Favor Attunements, for War Table Upgrades. As described in the source, the War Table opens after completing Awaken Queensguard, and Upgrades are earned from seasonal activities. This is the Season of Defiance description.