it's a amazing how much damage a priest can do, only if you choose to fully spec into the shadow priest build, when you know your on the right track when you get down to the point, where you can no long cast healing spells(this is an ability you can turn on and off thou!). They can really dish out damage with the best dps classes when they have had their talent points spent well. Throw a few into the Discipline tree, and the rest into shadow into the correct shadow trees, and you are what a warlock really should be, a machine that can blast out damage extremely fast, and in turn, using that damage to passively healing your entire group, it really does make warlocks jelous of what you can do!

Shadow Priest Build

Obviously, we need to cover how the points will be spent as a level 80 priest, ultimately you don't want to spend points randomly and hope you get it right. You need to ensure you have a plan upto level 80 to ensure you are on the right path the entire way. \Generally we suggest a 14/0/57 point spend (Discipline,Holy,Shadow).
We will be spending 14 points into the Discipline tree, as we don't want to be a squishy priest, no matter how good you are, if you go down in one or two hits, your pretty much useless, these 14 points will ensure you are a much harder target to take down:-
Twin Disciplines, 5/5
Improved Inner fire, 3/ 3
Improved Power Word: Fortitude, 2/ 2
Inner Focus, 1/1
Improved Power Word: Shield, 3/3

It's only cost us 14 points to ensure you are better off in battle compared to our other cloth wearing caster types, and they are 14 extremely well spent points.

Well now we'll talk about spending those points in the shadow tree, you know the ones that will make warlocks cry. By the time you have followed the talent tree build below for your shadow priest, you'll have large bonuses to your damage, get some seriously extended duration spells, have a much higher chance to hit and critial, and a couple of extra skills that will make you almost impossible to kill.
Spirit Tap, 3/3
Improved Spirit Tap 2/2
Improved Shadow Word: Pain, 2/2
Shadow Focus, 3/3
Improved Mind Blast, 5/5
Mind Flay, 1/1
Veiled Shadows, 2/2
Shadow Reach, 2/2
Shadow Weaving, 3/3
Vampiric Embrace, 1/1
Improved vampiric Embrace, 2/2
Focused Mind, 3/3
Mind Melt, 2/2
Improved Devouring Plague, 3/3
ShadowForm, 1/1
Shadow Power, 5/5
Improved Shadowform, 2/2
Misery, 3/3
Vampirit Touch, 1/1
Pain and Suffering, 3/3
Twisted Faith, 5/5
Dispersion, 1/1
When you see a full spec'd shadow priest on the battle field, you should be worried, just like they'll be worried about seeing you. In the end you can deal a lot of damage quickly, that continous on over time, even if they get you, you'll still get them, long after your dead.