Wall-o-text incoming.
Here is how I usually start out on a fresh pull (assuming 0 Shadow Orbs):
Shadow Word: Pain (SW: P), Devouring Plague (DP), Vampiric Touch (VT), Mind Flay (MF) until Shadow Orb.
Upon gaining Shadow Orb, Mind Blast (MB) for Empowered Shadows(ES).
Is VT/DP about to drop? Refresh (supposedly it's a dps loss if you refresh them early to give them the boosted effect from ES after the initial casting, so use MF until they're about to drop.)
If VT/DP has plenty of time: MF.
Target <25% / low on mana / moving and dots don't need to be refreshed? SW: Death (Masochism should be in your spec for mana returns and reduced self-damage from SW: D)
ES about to drop? If you have orbs, then MB, else MF to get orbs (generally you'll be able to generate at least 1 orb prior to ES dropping, you want to try and keep 100% uptime on ES.)
3 orbs? If Mind Blast is off CD, then Mind Blast, unless ES is already up with enough time to refresh any dots that are about to fall off.
In short, my priorities:
ES
VT
DP
MB (3 orbs)
MF
SW: D (can be moved up in cases stated above)
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Few key points:
On single target, once you've cast SW: P the first time, you shouldn't have to hard cast it again (MF should be refreshing it via talents.)
More often than not, Shadow Fiend is utilized to boost dps more than for mana returns (though if you KNOW you have mana issues, sometimes it's better to save it until your mana is below 90%, since it's a larger dps loss to use Dispersion mid-fight unless you're in a transition phase, but a glyphed SW: D is great for mana gains.) Dark Archangel should be used liberally (unless there is a "blow cds" part of a particular fight, then you'll likely want to make sure it's up for this) as it's a great dps increase + returns 25% of your mana with 5 stacks of Dark Evangelism.
For multi-target, generally you'll dot everything with VT (and SW:P if your mana can handle it) followed by Mind Sear. I prioritize the dots over Mind Sear since it's generally more damage per mana/cast time. Just keep in mind, you'll likely run out of mana on large trash pulls regardless of which direction you take (and focus firing is a valid choice for higher HP trash.)
MF and MS ARE considered dots and receive the effect of ES.
On trash, if they aren't going to last long enough for your dots to get a full effect... use Mind Spike/MB. Don't bother with dots on these mobs since MS removes them anyway.
Dot clipping no longer exists, so you want to try and refresh your dots as close to them being about to drop as possible without actually letting them fall off (unlike a hot healer, which wants things to drop off to get the full effect, dots continue to tick, instead of resetting when you "clip" them, hence the desire to refresh prior to them dropping.)
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Spec:
I can't link a spec from work, but you can check my spec: Schatze - Scarlet Crusade (US)
Feel free to ask any questions you have about my choice of talents.
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Glyphs:
Prime - SW: P, SW: D, MF
Major - Really up to you, Spirit Tap is suggested (though unless you're good at sniping KBs with SW: D, it's highly situational.) I personally use Psychic Scream (for limited CC), Psychic Horror (limited CC, disarm is useful for damage mitigation), and Spirit Tap (more for farming than anything.) It's suggested to pick up Mass Dispel if you find yourself on dispel duty.
Minor - Your choice.
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Gems:
Red - Int (Brilliant)
Yellow - Int/Haste (Reckless)
Blue - Int/Spir (Purified if you want to reach the hit cap, which isn't necessarily mandatory. Otherwise just use Brilliant.)
Meta - Burning Shadowspirit Diamond
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Enchants:
You'll want Int/haste enchants whenever possible.
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Reforge:
There is quite a bit of debate, but haste seems to be the clear winner after Int (based on sims, but in reality if you aren't able to notice your dots missing rather quickly, then hit/spir definitely becomes more important.) After those, Crit/Mastery are roughly equal. A quick note about hit/spirit, if you have a healing OS (which you said your guy is Disc) then spirit is generally better than hit so you don't have to switch gear between specs, but hit gained from spirit doesn't transfer to your Shadow Fiend, which could potentially hurt mana gains/bonus dps.
Hopefully I'm not forgetting anything.
