After clearing 25man raiding up to Malygos, I felt doing a compendium of (shadow)priest specific tips and tricks regarding use of abilities , glyphs and talents.
I'll be mainly talking about 25man, but if you got 10man suggestions I'll make note of them as so.
Nothing special, If mana is a problem the best time to use Dispersion is when positioning yourself between the Lava waves.
- Arachnid Quarter: Maexxna:
You can using Dispersion during Web Spray
- Plague Quarter: Heigan
Using Dispersion during the dance might save your life. You'll have very big trouble using Shadowfiend since on the floor your shadowfiend would have to dance as well, and on the platform there's a constant AoE around him.
Shadowfiend glyph recomended.
- Plague Quarter: Loatheb
With the changes to Shadowform your DoTs will tick for 50% more when you get the 50% crit from the Fungal Creep buff.
- Military Quarter: Instructor Razuvious
The adds have been fixed so your Mind Control cannot resist. Equip some extra stamina gear if you got it to avoid being killed instantly if MC gets dispelled before time.
- Military Quarter: The Four Horsemen
Improved Shadowform is a must on this fight. Depending on how your guild setup the groups, it's likely you'll be standing in front of Thane Kor'Thazz who'll do Meteor and causing pushback.
When you get to the point of having to DPS Sir Zeliek and Lady Blaumeux you'll regreat dearly if you haven't invested in [item=42406] - Long range is to key to easier spreading out and overall lower raid damage.
- Construct Quarter: Patchwerk
If you haven't started to use the new Mana Potions, now is the time. You'll get to blow all your cooldowns here, so use them early.
- Construct Quarter: Grobbulus
Using shadowfiend can be tricky on this fight, but there's a high chance of movement so Dispersion again comes in handy.
Again a fight where you'll start to cry if you haven't invested in the [item=42406].
- Construct Quarter: Thaddius
Shadowfiend don't get affected by Polarity Shift.
- Frostwyrm Lair: Sapphiron
Get the Mind Flay glyph for easier spreading out. Using Dispersion while behind a icecube is efficient.
Renew SW:P before he starts to cast Ice Bolts to ensure it last over the Frost Breath in case you got Icecubed.
- Frostwyrm Lair: Kel'thuzad
Guardian of Icecrown cannot be shacked.
Bring cake for the druids, and arrange a Innervate around 30%.
Use Dispersion during the first Vortex (it works while you're flying around).
Use Levitate when you're about to land to avoid fall damage. (Get [item=43370] if you haven't.)
And you should have enough mana to PW:S during the second and third Vortex to assist your healers. If you're a herbalist, pop Lifeblood during the Vortex.
Try to use your shadowfiend just before the entry to Phase 2 since it gets tricky to get optimal mana from it there.
In phase 3, just play your dragon right. That means, stack the Engulf in Flames debuff as much as possible by using 1-2 combo points and then the finisher.
Ofcource keep your drake alive, use Blaze if you lack energy for the shield. Nothing biggie here.