Dispersion

Dispersion is a defensive priest ability that is available via the highest talent of the Shadow tree. Dispersion protects the priest by reducing the damage taken by 90% for 6 sec. Meanwhile the priest will regenerate 6% mana every second up to 36% of mana in total. Due to its relative low cooldown and free cast, Dispersion is an excellent regeneration spell. Dispersion can be cast while stunned, feared or silenced, which makes it very useful in PvP as well.

It has many uses in PvE besides just regenerating mana, you can also use it to soak large (AoE) hits that would reduce a fully geared tank into a red stain on the floor.