We've datamined the Priest Tier set bonuses for Patch 9.2 Eternity's End! This includes the 2 and 4 piece bonuses for Discipline, Holy, and Shadow.
Discipline Priest Tier Set Bonuses in Patch 9.2 NEW (2p) Casting Power Word: Shield or Shadow Mend has a 15% chance to make your next cast of Shadow Mend have no cost or cast time. NEW (4p) Power of the Dark Side increases the effectiveness of Penance by an additional 45% and is triggered when Power Word: Radiance is cast. Read more on wowhead.com