Everybody loves spellthread! Use Runic Spellthread if you have Spellstrike, Mystic Spellthread otherwise.
Vitality, though it's not a big deal. Probably put it on either of the above, but it's a higher priority to enchant other slots and get yourself gemmed out.
Unlike most classes, you may find yourself not equipping the Violet Signet until Revered, Exalted, or ever, depending on what you already have; it has a fair amount of spell crit, which is sub-par for you. - Ashyen's Gift (Cenarion Expedition exalted). Simply awesome. - Seal of the Exorcist is quite strong, even though the resil does nothing for you in raids. Besides, what else are you going to spend your Spirit Shards on? - Band of Arcane Alacrity (Vexallus, Magister's Terrace). I didn't mention spell haste when going over stats, because it's hard to find pre-T6, but make no mistake, it's quite nice. - Seer's Signet (Scryer exalted) - Cobalt Band of Tyrigosa (Nexus-Prince Shaffer, heroic Mana-Tombs)
Ring enchants, of course, are enchanter-only; if available, you'll want Spellpower.
Alright, that ought to be enough trinkets; use the best two of those that you can get your hands on.
A main-hand, along with an off-hand, is almost certainly going to be better than a staff.
- Cudgel of Consecration (Kael'thas, Heroic Magister's Terrace). You may have to fight a Paladin tank, or other casters, for it, but it is a thoroughly excellent weapon (better than anything pre-T5)
- Merciless Gladiator's Gavel/Spellblade. When season 4 goes live in two weeks, this season 2 stuff will be available for honor. Nobody knows exactly how much honor, but I'm guessing it will be on par with what the season 1 stuff is currently selling for now (25,200 honor and 20 EotS marks). There's no difference between Gavel and Spellblade except appearance, so pick the one you think looks better. If you really can't decide, pick the Gavel; it has a higher maximum durability.
- Eternium Runed Blade (BoE blacksmithing, ~1500g)
- Gavel of Unearthed Secrets (Lower City exalted)
- Sky Breaker (Avatar of the Martyred, Heroic Auchenai Crypts)
Major Spellpower! We love spellpower here at Shadow Priest Central. Soulfrost is actually better, but it's shockingly expensive. I would put it on the Cudgel of Consecration or a Merciless Gladiator's weapon, but not anything less.
But seriously, just get the Badge one. This goes for the offhand, too.
You've probably heard this before, but let me tell you again: ignore the socket bonuses. In the vast majority of cases, you're just going to want to put Runed gems in everything; it won't be worth socketing yellow or even (shudder) blue gems to get +2 int or whatever. Obviously there are a few exceptions, so look at the socket bonus, but most of the time just toss in the red gems.
I'd advise putting Living Rubies in most of the stuff on this list, though Blood Garnets will suffice for the lower-quality items. Do not leave sockets open; you're just throwing away stats if you do that. Put in Runed Tourmalines if you really have to.
And there you have it: some lovely and relatively easy to obtain gear for entering the halls of Karazhan in Shadowform. You can get by with less, especially if your raid is well-geared and/or well-played, but I think these items are good to aim for. Have fun, and remember to keep Vampiric Touch up at all times.
In closing, I would like to give a massive shout-out to ShadowPriest.com, the best place I've found for Shadow information. The "Best casual gear available" list especially contains much of the information on which this guide is based. The Elitist Jerks thread "Shadow Priest 101" is also a must-read.
This concludes my guide to gearing a Shadow priest for Karazhan; here's part one. Spiritual Guidance is our Priest column -- Matt Low will fill you in every Sunday on all things of the cloth. And for even more Priestly goodness, check out the Priest guides section of our directory.