Let's say you are a fire mage in Shadow Labs. Your party has arrived at Murmor, the leader has explained the fight and you have been assigned a pillar. You give your tank a few moments to gain agro and open up. You cast five scorches followed by a stream of fireballs. Every 30 seconds you scorch again. You may have to pause to avoid a sonic boom or slow down to temper your threat. But essentially you just carry on. The worst that could happen is the scorch debuff drops off and you have to start again. Life is blissfully uncomplicated.
Not so for the shadow priest. Shadow priests have to juggle an irritatingly complex range of spell durations, casting times and cooldowns in order to maximise our damage output. A resist, some lag or the need to move simply slow a mage down; they make a shadow priest want to scream.
In a typical fight opening, it will take you 4.5 seconds to apply your dots and debuffs — 1.5 seconds for vampiric embrace (global cooldown), 1.5 seconds for vampiric touch and 1.5 seconds for shadow word: pain.
Now you have to decide what spells to cast while they take their course. Your key priority is to ensure maximum uptime of your dots without overlap, so the duration of those spells will determine your next move. Vampiric touch lasts 15 seconds; shadow word: pain, if talented (and it should be), lasts 24. You need to start casting vampiric touch 1.5 seconds before it is due to expire, which gives you 12 seconds to play with.
The easiest thing to do in that time is cast four mind flays, which last 3 seconds each. Then you can reapply vampiric touch at exactly the right moment. You then have time for another three mind flays before having to reapply shadow word: pain. Back to mind flay ad nauseum. This is a very mana efficient spell rotation and will serve you well in long fights where threat is an issue or dps is not.
It sounds simple enough, right? But even juggling just these three spells is problematic. The number of mind flays you can cast in-between reapplying dots varies every time and sometimes they don't fit in the gap at all. It gets harder when you want to weave mind blast and shadow word: death into the mix too.
Your spell rotation becomes a matter of determining the length of time to your next dot application, factoring cooldowns and avoiding dot voids and overlaps. From this point forward you will need the mathematical powers of a savant.
Smart readers out there will already have spotted other sabats in my spell casting cogs. Lag can play havoc with efficient spellcasting, as can certain debuffs, the need to move frequently during a fight, or a pause to reduce threat.
So shadow priests have very little in the way of spell rotations. This is what keeps face melting so interesting, and what makes it a bit more varied and spontaneous than other magical dps. But it's also one reason why some shadow priests may be performing below their potential (me for starters).
The good news is Blizzard appears to be aware of our pain and looking at a few options for the expansion. In the meantime, all we can go on is priorities, a concept which the dwarf priest established so beautifully in a post a few months ago. If you really want to know which spell to cast, take a look at his flowchart.
And as you should have realised by now, you can't perform your function competently without a solid dot timer. Intuitively, I used dotimer. A cast bar mod will also come in handy — Quartz, part of the Ace framwork, incorporates timers and usefully shows me things like lag and adjusted cast times.
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