During Burning Crusade the Shadow Priest was a welcome addition to any group if only for their ability to act as a mana battery. While they initially dealt competitive damage their scaling left much to be desired, and their damage fell behind in the later stages of the expansion. They were also lacking any form of area of effect damage, making them a spectator in any AoE fight. Blizzard has corrected these two flaws with Wrath of the Lich King and turned the Shadow Priest into a respectable damage dealer while giving him some strong AoE spells. This one is learned at level 75. The spell is not learned via talents, making it available to any Priest spec. It does no daamge to the current target, but damages any other target within the vicinity - a small constraint hardly reducing the effectiveness of this spell.
The other goal Blizzard has reached with Wrath of the Lich King was making the Shadow Priest scale with every stat available on his gear. While critical strike rating hardly had any effect in the past as damage over time spells were not able to crit, those spells have now been modified to gain an effect from critical strike rating. Spirit has been made more interesting as well, and the corresponding talent which improved your gains from spirit whenever you killed a target ( a rare occurrence in a raid) can now be triggered by a critical strike.
In order for the additional damage to not blow up the mana regeneration of the Shadow Priest its amount is no longer tied to the damage dealt but now scales with the total mana of any affected raid member. Additionally the Shadow Priests Monopoly on the mana battery ability has been removed and other classes like Frost Mages and Survival Hunters are now able to restore their group member's mana as well.