Spell priority is a bit of a misnomer -- there's really no set spell priority list that works in all situations. (That's just part of what makes shadow priest a challenging class to play!) In general, though, the most important thing is that you keep all three damage over time spells active on your main target as much as you can.
Beyond that, well ... let's just take a quick look at the individual abilities.
- Mind Spike has two uses. It's a terrific opener -- it's a quick cast, and it buffs your next cast of Mind Blast. For that reason, it's also a powerful burst ability. Word of warning, though: It does erase any and all existing DoTs on a target, so its use is limited to those two situations.
- Shadow Word: Death is the shadow priest's Execute. What does that mean? Well, if your target is above 25% health, this spell isn't worth casting unless you have no other options while moving. Once the target dips below 25% health, though, its power triples, making it an exceptionally powerful spell. You should aspire to finish off all opponents with a cast of SW:D if you can -- doing so will proc Spirit Tap (if glyphed), a terrific mana-regen buff. Be careful, though -- casts of SW:D that don't kill its target will cause you to take a chunk of damage.
- Vampiric Touch is arguably our most important damage over time spell because of its ability to proc Replenishment. It has the shortest duration of all your DoTs (15 seconds) but the highest total damage output. Even on trash pulls, it's a good idea to keep Vampiric Touch up and active at all times.
- Shadow Word: Pain is invaluable in long fights. You only need to cast it once; it will continually refresh itself via Mind Flay (given you've invested talent points in Pain and Suffering like I told you to). As an added bonus, ticks of SW:P have a chance to proc a Shadowy Apparition for extra damage (again, provided you have the talent). It has a 15-second duration, so cast it whenever you feel a fight will last at least that long.
- Devouring Plague has the longest duration of all our DoTs (24 seconds). It's also the most expensive, mana-wise, to cast. Still, it will provide a chunk of damage up front and provide you a small amount of bonus self-healing, and it's an instant-cast ability.
- Mind Blast often crits for high five-digit damage once you hit level 85; by the end of the Cataclysm expansion, Mind Blast will easily achieve six-digit crits. It has tremendous synergy with Vampiric Touch (proccing Replenishment) and can further proc Empowered Shadow, our mastery bonus. In general, you should cast Mind Blast whenever it's off cooldown, provided all three DoTs are active.
- Mind Flay is the shadow priest filler spell. If you don't need to refresh your DoTs, and if Mind Blast isn't available, you should be casting Mind Flay.
- Mind Sear is the shadow priest's area of effect spell. It damages all enemies around the target, but notably, it doesn't damage the target itself. In Wrath of the Lich King, Mind Sear was quite powerful; in Cataclysm, it's underpowered. This should be cast only in situations in which there are multiple, low-health adds. Where possible, only cast Mind Sear on the tank as a target so the searing wave hits all the enemies.
Beyond these, you should use your Shadowfiend and Dark Archangel abilities whenever they're available (provided you need the mana regen). This means that for long boss fights, you should cast Shadowfiend almost immediately after applying DoTs to your target.
In Cataclysm, gemming is simple. For your meta gem, you should use the Burning Shadowspirit Diamond, though the Chaotic Shadowspirit Diamond can work in a pinch. Both require that at least three red gems be socketed to activate.
The rest of your gem slots should almost always be filled with the 40 intellect Brilliant Inferno Ruby, regardless of socket color. There is an exception to the rule, of course: For especially lucrative slot bonuses (greater than 20 intellect), use the Reckless Ember Topaz for yellow slots or the Purified Demonseye for blue slots.
Of course, jewelcrafters should socket their three jewelcrafter-only 67 intellect Brilliant Chimera's Eye gems as soon as they can.
At level 25, shadow priests get access to their first prime, major, and minor glyph slots; they get another set of three at levels 50 and 75. That's a total of nine slots, but don't fret about being overwhelmed with choices -- there are only a few glyphs worth taking.
As for the prime glyphs, there are only four of real value to shadow priests. As raw DPS boosts, the Glyph of Mind Flay (a 10% buff to a frequently used spell) and the Glyph of
Shadow Word: Pain (again, a 10% buff to a frequently used spell) are easy choices. As for your third slot, you have a choice between the Glyph of Shadow Word: Death, which allows you to immediately recast SW:D if the first casting fails, and the Glyph of Dispersion, which cuts the cooldown on Disperion from 2 minutes down to 1 minute and 15 seconds. I prefer the former, though the latter is quite useful situationally -- it's your choice.
Three major glyphs stand out from the crowd. The Glyph of Inner Fire boosts the spell's bonus armor effect by 50% and is terrific for leveling; the Glyph of Psychic Scream converts a dangerous-to-use panic button to a safe, 8-second burst of crowd control; and the invaluable Glyph of Spirit Tap allows you to regain 12% of your max mana with a SW:D-caused kill. There are a few more options -- some shadow priests have been experimenting with the Glyph of Holy Nova, while others like the Glyph of Psychic Horror.
There are only six minor glyph options in total, and almost all of them make viable choices. Personally, I like Glyph of Levitate, Glyph of Fortitude, and Glyph of Shadowfiend.
12. Endgame enchants
Head The best head enchant, Arcanum of Hyjal, provides you with a luscious 60 points of intellect and 35 points of critical strike. The enchant is bind-on-account; to get it, at least one of your toons needs to be revered with the Guardians of Hyjal faction.
Shoulders Any character can use the Greater Inscription of Charged Lodestone, provided you purchase it on a toon that is already exalted with Therazane. It provides you with 50 intellect and 25 haste. Scribes have access to something better, though -- Felfire Inscription, which provides 130 intellect and 25 haste at the cost of four Blackfallow Inks.
Cloak Tailors can take advantage of the Lightweave enchant, which often procs to increase your intellect by 580 for 15 seconds. For the rest of us, there's Enchant Cloak - Greater Intellect, which provides 50 intellect at all times.
Chest For now, I recommend picking up Enchant Chest - Mighty Stats enchant, which increases all your stats by 15. Technically, Enchant Chest - Peerless Stats is better; it increases all stats by 20. Unfortunately though, the cost is prohibitive, to say the least -- the latter will cost you thousands of gold.
Wrist The cheaper option is Enchant Bracer - Speed. It buffs haste by 50 and is often available on the auction house below cost. The second option is much more expensive but much more powerful: the 50 intellect Enchant Bracer - Mighty Intellect. The latter requires two expensive Maelstrom Crystals to make, so unless you're a hardcore raider, take the haste enchant. Leatherworkers have a terrific plus-intellect wrist enchant, but ... what shadow priest in his right mind would be a leatherworker?
Hands For most of us, the plus 50 haste Enchant Gloves - Haste works great. Engineers can also Synapse Springs to their gloves in addition to the haste enchant. Synapse Springs provide an on-use burst of 480 intellect for 10 seconds.
Waist Purchase an Ebonsteel Belt Buckle off the auction house and put a plus-40 intellect Brilliant Inferno Ruby in it.
Pants Powerful Ghostly Spellthread provides 95 intellect and 55 spirit, making it the best pant enchant available. If that's too expensive, you can save money with the cheaper 55 intellect and 45 spirit Ghostly Spellthread option. Tailors get off even cheaper yet -- the tailoring-only Sanctified Spellthread option provides the benefits of Powerful Ghostly Spellthread at a cost of only three Eternium Threads.
Boots For shadow priests on a budget, I recommend Enchant Boots - Earthen Vitality, which provides 30 stamina and a 8% movement speed buff. Alternatively, you could take Enchant Boots - Haste instead, but you lose the valuable movement speed buff. The best option in terms of DPS is Enchant Boots - Lavawalker, which provides the movement speed buff of Earthen Vitality with 35 points of mastery, but it's quite expensive at the moment.
Finger The only people capable of enchanting rings are enchanters -- it's a special perk to the profession.
Off-hand If you have an off-hand item, you may as well enchant it. You have only one option here: the 40 intellect Enchant Off-hand - Superior Intellect.
Weapon If you're toting around a one-handed item, your least expensive option here is Hurricane. It offers a proc of 450 haste for 12 seconds. If you're using a two-handed staff, the old Wrath staple Enchant Staff - Greater Spellpower is technically better than Hurricane (and by a decent margin, too). If money is no object, you can take a look at Power Torrent, which offers a 500 intellect for 12 seconds proc ... but it'll set you back over 10,000 gold (at least on my realm).
Are you more interested in watching health bars go down than watching them bounce back up? Think it's neat to dissolve into a ball of pure shadow every few minutes? Hunger for the tangy flesh of gnomes? The darker, shadowy side of Spiritual Guidance has you covered (occasionally through the use of puppets).