For raiding, the glyph of shadow and glyph of mind flay are no-brainers. But since the third glyph slot was opened up with the base range increase of mind flay, options for the third slot are a little less cut-and-dry.
We have four key contenders. None will do a great deal for your dps, but there's probably a good argument for any one of them. This is just my view...
Glyph of shadow word: pain
This glyph returns one per cent of base mana each time shadow word pain ticks, which for priests at level 80 means 38.63 mana every three seconds (rounded up or down).
If we assume 100 per cent uptime of shadow word: pain, it will return 772.6 mana a minute on one target, or about 64 mp5. That doesn't seem like much, but when you review your in-combat mana regen, 64 mp5 is probably a significant percentage of what you are currently running with.
I think this glyph provides a reasonable amount of mana given that it's a passive drip you can set and forget, but as a base mana effect it doesn't scale, and it's small enough to question whether it is needed at all. It's a slightly better deal if you can keep more than one shadow word: pain rolling at a time, but the boss fights in which you can do that are few and far between. So the question is, do you need the mana?
The answer may well be yes, but I would not even consider this glyph until you are running with 2/2 veiled shadows and know that a three-minute shadowfiend is not cutting the mustard.
Best for: a reliable, passive boost to mp5
Misery rating: ★ ★ ★ ☆ ☆
Glyph of dispersion
This glyph, generally recommended by experts, reduces the cooldown of dispersion by 45 seconds, a huge chunk for a spell that already has a short two-minute cooldown. For that, you get more mana and more chances to cheat death. Or you might just forget to use it and get no benefit at all.
As an active ability that silences you for six seconds, the mileage of dispersion already varies wildly by fight to fight, and by player to player. Most players are naturally reluctant to cast it while they could otherwise be doing damage. So it becomes a useful mana pot while on the move and something to help you get out of trouble and give your healers a break when required.
On mana alone, the glyph is far more effective than the glyph of shadow word: pain, but only if it's used. With a mana pool of 20k, popping dispersion every one minute 15 seconds will net you 5760 mana per minute (480 mp5). But even unglyphed, dispersion provides a potential of 3600 mana per minute (300 mp5). Managed effectively, a talented shadowfiend and an unglyphed dispersion should be enough to keep you going under any circumstances.
Where this glyph really shines is in its damage reduction, and this again is something very hard to quantify. You're not going to need it every one minute 15 seconds like clockwork. Some boss effects even bypass dispersion (and one of these days I'm determined to find out which ones and tell you about them!) And in many situations, your healers are going to be on the ball and ready to keep you up no matter what. But it's a safety net for stupidity if you should find yourself in a tight spot, and with a shorter cooldown there is more room for error in popping it at the wrong time.
Best for: survivability, maximum control
Misery rating: ★ ★ ★ ★ ☆
Glyph of shadow word: death
With this glyph, targets below 35 per cent health take an additional 10 per cent damage from the spell. It's a poor comparison to most other execute-style abilities in the game, but it's the best we have. So is it any good?
Well, no, I don't think so. In a five-minute fight, assuming no movement or interruptions to casting, you would be able to cast shadow word: death a maximum of eight times during the execute window — and that in itself does not allow for a much quicker burn below 35 per cent, which with mages and locks and shammies kicking around you can almost guarantee. Even on an eight minute fight, you're looking at only 14 or so opportunities.
But all things considered, this is the only option for the third slot that's going to boost your dps, and for some min-maxers that might be the most important consideration. If shadow word: death is something you cast frequently anyway, give it a go.
Best for: min-maxers
Misery rating: ★ ★ ☆ ☆ ☆
Glyph of mind sear
This glyph increases the radius of mind sear by five yards. It certainly sounds like it would make spamming our killer aoe a little easier, but the conditions under which you would gain any benefit from this glyph seem so specific I'm not sure where I could use it. You need a boss fight which includes packs of four or more mobs that need dpsing down and which are close enough to be in range of the glyphed spell but not quite close enough for it unglyphed.
Well, to be fair, it doesn't have to be a boss fight, you might want to use it on trash. But who would choose a glyph for trash over one for bosses?
The glyph was introduced in a recent patch to rectify a clear dearth of glyph choices for shadow priests. Most of us were still gimped by the glyph of mind flay at the time and discounted it. Sadly, I'm not sure it's strong enough even now to justify the slot, but it might fill a very specific niche for a small number of shades. the best argument I can think of for equipping this glyph is if you were to swap it in and out like consumables where required — which is a perfectly valid strategy if you have access to cheap glyphs, but I usually have enough to think about before bosses without making sure I'm sporting the right glyphs.
Best for: trash
Misery rating: ★ ☆ ☆ ☆ ☆
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