A ground-placed dome providing Damage Reduction to anyone standing inside. It has the strongest and longest DR of any raid-wide cooldown in the game, with the requirement that the raid must stack up.
If stacking isn't possible, the priest can spec into Luminous Barrier, which is a weaker option but still provides some utility.

Assign when the raid can stack and mitigate high amounts of damage in a 10-second window. It can also be useful when a group of players is taking lethal damage, reducing their chances of death.