Aura Mastery empowers your active Devotion Aura, providing a 12% raid-wide damage reduction for the duration. It also enhances other auras (such as adding to Crusader Aura, Retribution Aura, and Merciful Auras) but the Devotion Aura damage reduction is what makes it a raid cooldown.

It is raid-wide damage reduction, making it uniquely strong when other DR requires a stacked raid.

Cover damage events where the raid is stacked with the ground-based DR options first (Anti-Magic Zone, Darkness, Power Word: Barrier), and save Aura Mastery for spread phases, soaks, or anything where the raid can't collapse onto a ground effect.

With the short duration, time the cast tightly to the damage spike rather than pre-casting too early.