Quite often, I've heard people say that Disc or Holy are the best for leveling 1-40, but in my experiences, I've found that to be false. Yeah, you CAN level as Holy or Disc, but it's slower and less efficient, and continues to get worse in comparison to Shadow, the higher level you get. Shadow provides quite a few very good benefits that simply aren't matched by the healing specs.
BEFORE YOU BEGIN
Wands are absolutely critical to leveling a Priest. They make your life so, so much easier by saving you tons of mana. You can find the Enchanting-crafted wands for fairly cheap on the AH. The level 5 and level 13 wands should only be about 5-15 silver each, due to really cheap mats. Sadly, their damage was not buffed until patch 1.8, but when that patch does drop, the four Enchanting wands all become super OP and do almost double the damage of similar level green wands.
You'll definitely want to make a pilgrimage to BFD around level 24-25 to do the quest to get the Gravestone Scepter, it's really nice. I also found that the blue BoE wands were not too expensive either. I bought Earthen Rod, Wand of Allistarj and Wand of Arcane Potency for about 5 gold each.
Also, don't worry about being Shadow for healing dungeons. You can heal them just fine, so long as the tank isn't a complete scrub. I healed every single dungeon (except WC and RFC) as Shadow, all the way up to and including Sunken Temple. No dungeon really posed much of a challenge, healing-wise. But bring a couple mana potions along, just in case shit hits the fan. Once you get to BRD though, you'll definitely want to be in a healing spec. You might get away with shadow healing the first half of the instance, but the latter half is much more difficult, in terms of healing. Especially the last couple of bosses.
SPECS
Pretty straight forward; most of the talents are self-explanitory. Start off with 5 points in Improved Wands, then 5 points in Spirit Tap and continue down the Shadow tree from there.
At level 40, you'll want to respec and go full Shadow to get Shadowform, then add 5 points back into Imp Wands and continue down the Disc tree from there. It's not absolutely necessary to do this, but I found it to be easier.
Here's the builds:
Level 20
Level 30
Level 40
Level 50
Level 60
GEARING
In general, you'll want to stack spirit. With Spirit Tap giving a fat +100% spirit bonus, you can easily recoup most of your mana after killing a mob, and the more spirit you have, the more mana you get back after each mob.
You really won't find much damage gear while leveling. Almost all of the green gear you'll come across will be raw stats. Just take whatever Int/Spirit pieces you can get. Stam/Spirit and "Of the Whale" pieces are good if you want to have higher health for world PvP. Can be pretty nice to not get 100-death'd by a rogue stunlock.
A couple of shadow damage pieces you can get early on are the Shadow Hood and Hands of Darkness. Both crafted by Tailoring, but they shouldn't be TOO too expensive; their mats are only about 3-4 gold each.
There is also the Shadoweave set which requires levels 37-44, but a lot of pieces are just stamina+shadow damage, with no Spirit on them. A couple of the pieces have Spirit, and a couple of the pieces have Stamina. Go for the ones with Spirit on them. If you want to craft the whole set though, it's about 115 shadow damage total. Pretty nice. Shouldn't cost too much either, as long as you get the 5 stacks of Mageweave cloth yourself. I find that the various spider silks are pretty cheap on the AH.
ROTATION / GAMEPLAY
The pre-Shadowform rotation is:
1. Power Word: Shield before you pull
2. Mind Blast (use Smite during the levels before you get Mind Blast)
3. Shadow Word: Pain
4. Wand the mob until it's dead
It's kinda slow killing, but wanding allows you to regen mana, and you'll often be able to go 8 to 12 mobs (depending on int+spirit) before having to drink, compared to other casters having to drink every 2-3 mobs. Shield + MB + SWP will cost around 15-25% of your mana, but with wanding and Spirit Tap, you'll regen most or all of it back by the time you're at the next mob.
The post-Shadowform rotation is:
1. Mind Blast
2. Shadow Word: Pain
3. Mind Flay
4. Rank 1 Psychic Scream
5. Mind Flay and/or wand until the mob is dead
Now, why fear instead of shield? Because rank 1 fear costs much less mana than shield, and gives the same desired effect: allowing you to mind flay without spell pushback. However, if there are enemy players around, it might be wiser to use shield, in case they attack you.
You can still use the pre-shadowform rotation at 40+, but I find that the strength of wands decreases quite a bit past 45 or so, even if you're using 40+ blue wands. It's still nice to have a wand for finishing off mobs though, so if you can find an Allistarj's for <5 gold on the AH, go for it.
TIPS / TRICKS
-Mind Control can be very useful for dealing with situations where there are 3 or 4 humanoids close together. If you pull with Mind Control, the other mobs will start attacking your mind controlled target. With a bit of good timing, you can drop the mind control when the mob is at low health, then finish it off with a quick Mind Blast or something, and run away to make the other mobs reset.
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That about covers it for Shadow leveling. If there's anything I missed, or additional tips/tricks you want to share, just say so.
