Shadow priesting. It's really important. See, if you can't dps, you can't kill a boss. If you can't kill a boss, the raid doesn't succeed. And if the raid doesn't succeed, it's quite depressing. And since depression is something that's often considered a "bad thing," it's important to know how to shadow priest stuff like the best of 'em. Anyhow, this is a guide how to deal damage as a shadow priest in an efficient, expedient manner. So, let's begin, shall we?

A: An Overview
A shadow priest is a ranged dps class. They provide replenishment, a buff that restores mana to the raid, and constant healing to their party through Vampiric Embrace. The tradeoff for this is a simple one that can often become a nuisance: Range. A shadow priest is limited to a 30 yard range on their "main nuke spell", Mind Flay. Luckily, they rely on their damage over time (DoT) spells in order to deal the brunt of their damage, which allows them to continue doing damage despite being out of range or otherwise incapable of casting. The average shadow priest's "rotation" involves applying every 'DoT' available to them, and maximizing their uptime while casting Mind Blast and Mind Flay in the meantime. This changes a bit once you have your 4 pieces of tier 10, but more on that later.

B: The Spec
A shadow spec relies on putting most of their talent points in the shadow tree, while putting the rest into discipline in order to remain mana-efficient. It looks something like this.

http://www.wowhead.com/talent#bVcbhZZGxfiRckbfqfzAo:qah

Simple, no? But let's go into it talent by talent.

Shadow
Tier 1
Darkness: 10% damage increase to your shadow spells. ... Okay, seriously? Do you have to ask? -.- GET IT.

Spirit Tap/Improved Spirit Tap: Spirit Tap will increase you spirit every time you strike a killing blow on a mob. This is awesome while soloing, because let's face it, less drinking time is nice. Improved spirit tap will allow your mana to continue its regeneration while casting when your Mind Blast/Shadow Word: Death crit. Since you need mana in order to cast, this will help greatly in the long run, unless you plan on using Dispersion every cooldown. Just get it, it's nice.

Tier 2
Shadow Affinity: Skip it. Threat reduction is not useful in today's high threat environment. If your tank can't hold off you, it's their issue. Further, you get passive threat reduction simply from being in Shadow Form, so... yeah. Skip! (Unless you're really having issues.)

Improved Shadow Word: Pain: Boring but practical. A straight 6% increase on one of your most consistent DoTs. Get it!

Shadow Focus: 3% bonus hit. This makes it easier to hit the 'Hit Cap', increasing your dps right from the get-go, and making it much easier to gear up. It also allows for your to prioritize other stats such as spellpower or crit over hit, to help out your dps once you're capped. The mana cost reduction on your spells is handy as well.

Tier 3
Improved Psychic Scream: Optional, but I like to get it in order to get Silence later on. More on that later, but this talent on its own is quite lackluster. I guess it'll help in PvP, and... er, Faction Champs? Yeah...

Improved Mind Blast: Less cooldown on Mind Blast in addition to adding a healing debuff on whoever is hit with it. Definitely should be gotten if you don't have your 4 pieces of tier 10. Once you get that, however, you won't be casting Mind Blast much. Or at all. You can skip it at that point, but... eh. None of the other options are overly attractive.

Mind Flay: Your nuke spell. Self explanatory: Get it.

Tier 4
Veiled Shadows: Many people skip this talent, but I beg to differ. This will bring your Shadowfiend's cooldown down to 3 minutes, which means you'll be able to cast it twice on most boss fights. The little bugger actually does a good chunk of damage, so... Yeah! I recommend.

Shadow Reach: Skip, unless you've freed points from Improved Mind Blast. In which case, the extra reach never hurt anyone. Another 6 yards for most of your spells.

Shadow Weaving: Um, yeah. Free 10% damage. Do want. Makes us have a bit of a ramp up time with our damage, but eh. I won't complain.

Tier 5
Silence: Formerly I wouldn't get this, but the ranged interrupt component gives it some raid use. It's very useful in PvP as well to shut down casters. In raids, tanks probably won't mind you getting a caster over to them. Only drawback is having to spent points in Improved Psychic Scream, but eh. Whatever. It's quite nice, if on a bit of a cooldown...

Vampiric Embrace: Twilight references aside, this is what allows you to heal your group while doing damage. Get it, we'll talk about it later.

Improved Vampiric Embrace: Extra healing on something you'll always have up anyway? Sure, why not? Not always useful, and adds a good chunk of threat to you. But let the tanks deal with that!

Focused Mind: Skippable, unless you've freed up points from Improved Mind Blast. Usually you have enough mana to get by, but... It can't hurt, right?

Tier 6
Mind Melt: Delicious free crit. Do get. Free damage for the win!

Improved Devouring Plague: Adds some direly needed instant damage to our arsenal, and ups Devouring Plague's damage overall. Definitely recommended, don't miss it!

Tier 7
Shadowform: Yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes. Yes. If you don't get this, just... go back to holy or disc and think about what you've done.

Shadow Power: Doubles your crit damage on your direct damage spells. No brainer there, get it!

Tier 8
Improved Shadowform: Lackluster, but there's not much else to get. Makes it so you don't have to worry about your spells getting pushed back as much. Also, you can fade out of snares. Not that great, but... hey, only 2 points.

Misery: Free spell hit for your group and for you. You'll only need 11% spell hit with both this and Shadow Focus, so do get it. Other people will thank you for it. To sweeten the deal, there's some extra spellpower thrown in as a little thanks for being so self-sacrificing. Get it.

Tier 9
Psychic Horror: PvP talent. Meh. Don't bother.

Vampiric Touch: Extra DoT. Yes. Allows us to supply Replenishment, at least until we get our 4 piece tier 10 bonus. But seriously, extra, quick DoT. Get it.

Pain and Suffering: You no longer have to apply Shadow Word: Pain after the first application, pretty much. I'm all for being lazy, personally! Frees up global cooldowns for damaging spells as well. Secondary effect... ... meh. Who uses Shadow Word: Death?

Tier 10
Twisted Faith: Spellpower from the otherwise useless Spirit. 20%, to be exact. Since Sprit is found on a lot of caster gear, this is pretty much free Spellpower. Do get. Also, an extra 10% to Mind Flay and Mind Blast. Nice.

Tier 11
Dispersion: A fairly saddening end to an otherwise awesome-filled tree. Free mana's not bad, and the damage reduction has its uses. But yeah, it's just... not as great as it could've been. Oh well. Do get it regardless.

Discipline
Tier 1
Twin Disciplines: Free damage for your instant spells. Shadow Words: Pain and Death, as well as Devouring Plague. Beats speccing into holy, anyhow...

Tier 2
Improved Inner Fire: Free Spellpower. Yay. Do get.

Improved Power Word: Fortitude: Lets you buff people more effectively. Ayup. Filler!

Tier 3
Meditation: This pretty much allows you to have enough mana to keep up your dps. Get it and get the hell out of disc.

C: Spells and Strategy
This isn't hard to grasp, so listen up. You have 3 damage over time (DoT) spells. These are Shadow Word: Pain, Devouring Plague, and Vampiric Touch. Your number one priority is that all three of these spells should be on your primary target.

You have 3 direct damage spells. These are Mind Blast, Mind Flay, and Shadow Word: Death. These spells act as filler. When all of your DoTs are on the target, cast Mind Blast. When Mind Blast is on cooldown and your DoTs are still on the enemy, cast Mind Flay. When you need to move and Devouring Plague does not need a reapplication and Shadowfiend is not on cooldown, cast Shadow Word: Death. The important thing to remember while moving is that when the Global Cooldown is not currently ticking down, you are wasting dps time. This isn't to say that you should ignore moving out of the fire in favor of more damage, but... make the most of it.

You have 5 buff spells. Inner Fire, Vampiric Embrace, Power Word: Fortitude, Shadow Protection, and Divine Spirit. All of these should be on you at all times. So yeah.

You have 1 AoE spell. This part is a little tricky: Mind Sear does damage to everyone except the target you've casted it on. Everyone within 10 yards, anyway. Cast it in AoE scenarios (duh). But what qualifies as an AoE scenario? If there are 1-3 mobs, you're best off sticking with putting DoTs on each individual target. In cases of 4+ mobs, go with Mind Sear. If there are 4 mobs exactly, it may be prudent to place your DoTs on the target you will hit with Mind Sear in order to keep up damage on that target.

You have 4 cooldown spells. Fade, Dispersion, Shadowfiend, and Power Word: Shield.

Fade is used to drop aggro, but when the effects wears off all aggro previously dropped will be returned to you. So please stop casting or switch targets if you need to cast this spell.

Dispersion is used for damage reduction and mana regeneration. It can be used to survive effects that would otherwise kill you, as it reduces the damage you take by 90% while active. You will also gain 36% of your mana back when you use it. If you need the mana from it, try to use it when you'd otherwise be prohibited from casting damage-dealing spells in order to maximize its effectiveness.

Power Word: Shield will absorb a set amount of damage on your target and apply Weakened Soul, keeping them from being shielded for 30 seconds. If you have a discipline priest in your raid, do not cast this spell unless told otherwise. They'll thank you for it. It can be used to prevent a bit of damage when needed when there is no disc priest in the group.

Shadowfiend: Your one and only damage dealing cooldown with a tied in mana regeneration aspect. The Shadowfiend should be used like a DPS cooldown. That is to say, use it whenever it's up. It'll deal a good chunk of damage. If you can, however, use it just before Heroism is cast so it will receive the benefits of the added haste. It will have a pet bar, so be sure to direct it to use its ability to increase its damage and keep it on your target.

To Review
Make sure you have your self-buffs. Apply Shadow Word: Pain, Vampiric Touch, and Devouring Plague. Cast Mind Blast when DoTs are all on the mob. Cast Mind Flay if no other options are available. Cast Shadow Word: Death if you must move and Devouring Plague does not need reapplication. Cast Shadowfiend if it is off cooldown. (Note: You will not have to worry about Shadow Word: Pain, as it will remain on the mob constantly thanks to Mind Flay)

DoT Clipping
This is what will make or break your DPS. When you apply a DoT, it will wait 3 seconds (before haste is taken into account) before doing its first 'tick' of damage. This is taken into effect even if the DoT is already active on the enemy. So if you apply the DoT before it has finished, you will have to wait another 3 seconds before its damage continues. Due to this, it is not good to reapply a DoT before it has finished its duration. You want it to land just as the previous application of the DoT falls off. It's better to reapply too late than too soon!

Four Pieces of Tier 10
Once you get your four pieces of tier 10, your Mind Flay will have its duration reduced by .5 seconds. This will make it so you can get 3 ticks of Mind Flay in roughly the time it takes to cast Mind Blast. This means you can drop Mind Blast from your rotation entirely, once you have the gear. This will be a huge boost to your dps, so be sure to get those Frost Badges spent immediately.

D: Stats
Pretty much every piece of cloth gear will have stats you'll want on them, but some are better than others. Your most important stats are:

Hit
Spellpower
Haste
Crit
Spirit
Stamina

The first and most important stat to have is Hit. You want to reach the hit cap, after which hit becomes the least useful stat. The hit cap is 11%, or about 289~ hit. Once you are hit capped, you'll want to focus mostly on Spellpower and Haste. Shadow, fortunately, has no "haste cap" as DoTs will never reach the point where they will run out faster than the Global Cooldown. So really don't worry about that. Spellpower and Haste are about equal, point for point. Critical Strike Chance (Crit) is your other important stat, though not quite as important as Haste or Spellpower. It's worth having, but not worth gemming for. Spirit comes in last in terms of DPS stats. It will increase your spellpower by 20% base, and glyphs and talents will up that amount somewhat.

Need to decide what gear to use? Think of this simple chart.

Spellpower = 1
Haste = 1
Crit = .75
Spirit = .5

This list isn't the end all, and does quite a bit of rounding. But it's good enough. And remember: Be hit capped!

Gems
Red: Runed Cardinal Ruby (Spellpower)
Yellow: Reckless Ametrine (Spellpower/Haste)
Blue: Purified Dreadstone (Spellpower/Spirit)
Meta: Chaotic Skyflare Diamond

E: Glyphs
There are only 4 glyphs to even consider using, and 3 that you should use.

Glyph of Mind Flay: Increases Mind Flay damage by 10% when target is afflicted by Shadow Word: Pain.
Glyph of Shadow: Non-periodic Spell Critical Strikes increase your spellpower by 30% of your total spirit.
Glyph of Shadow Word: Pain: Each tick of Shadow Word: Pain restores 1% of your base mana.

Yup. Use those glyphs. Glyph of Dispersion can be substituted in for Glyph of Shadow Word: Pain for Sindragosa to deal with PATHETIC MAGIC BETRAYAL.

Anyhow, that's pretty much it for being shadow. Questions? Ask! In game. Or post here. Either or!

~Ado K. Earthshadow
~Matt