One of the things that seems to be the newest hit in Shadowpriest design land is the “dot removal”. For one reason or another, after the relative faillure (read: very situational use) of Mind Spike, the addition of Shadow Word: Insanity seems to solidify the juxtaposed phasing of Shadowpriest abilities in a “Dotting up – Explode dots” model.
How it is today
Mind Spike as a spell hits as hard as Mind Blast, but removes all dots on the target. Properly talented, it is buffed by Shadow Orbs and itself will reduce the cast time of Mind Blast, effectively making the next one instant. The basic idea is that this spell will only be used when its Shadow Orb-buffed damage exceeds the damage your Dots do in that time plus the time it takes to reapply them all.
Currently, the only time when this happens is when you have small, easily killed adds that you cannot AoE down, or on Spine of Deathwing (once you have 4piece T13). The latter is because you will use all cooldowns on the Tendon phase, meaning Shadowfiend will give you a lot of Orbs, causing practically all your Mind Spikes and Mind Blasts to benefit from a full three Shadow Orbs.
In Mists of Pandaria
You won’t have the tier 13 bonus, unless they choose to roll it into a talent or glyph, which is highly unlikely. This removes the benefit of using Shadowfiend as a buff for Mind Spike. This means that Mind Spike will once again struggle to overcome the barrier of (Damage Done)>(Dots DPS + DPS loss from recasting dots).
Shadow Word: Insanity has a similar model, except that it will extinguish your dots and then increase its damage done by up to 100% per dot removed. The bolded part is important, because it is dependent upon how long your dots were ticking when they were removed. This means that the SW:I will deal the most damage at the last second of each dot.
The picture above is a basic model of how this would work (click it for a larger view); base damage of the spell is rather low (arbitrary “half a DoT’) but is boosted by the consumed DoTs. It has a break-even point, after which the SW:I will deal more damage that the DoT itself, but you’d want to wait a bit more to ensure you also “cover” the time lost recasting the DoT.
This is split-second timing over 3 dots (SW:Pain, Vampiric Touch and the currently languishing Devouring Plague) and their respective durations. Fortunately it’s instant-cast, so it only has to overcome the barrier of lost recasting the dots, which you would have to anyway considering you cast this spell close to the end of a dot duration. There are a few scenarios to think of as to the use of (and reason to introduce) this spell:
SW:Insanity as a cycle ender
Basically, using SW:Insanity as close as possible to the end of the dots, using it to clean the slate and reapply all dots. This comes down to casting this spell every 12 seconds or so, after which you spend 3-4 seconds reapplying them. This could be a DPS gain or loss – but it would take quite some maths to figure it out since the break-even point of SW:I is highly dependent on gear and scaling. It is, without a doubt, hideously mana inefficient if not used at the split second before a dot would expire needlessly – and with three dots this is likely the case.
SW:Insanity as Burst
If the target is going to die before the dots will do their job (a problem we Shadowpriests are well familiar with) then you can use SW:I to “explode” the dots, allowing you to at least get some damage in before the target dies. This will be mostly useful in 5mans, where dot classes suffer noticeably under more bursty classes. If a mob has 1 million HP, and your party members burst more than you, you end up at the bottom of the meters. It may not be important, but neither is it fun – so this is probably the best use for this spell.
New mechanic is new
A last scenario is not a scenario for the uses of the spell, but the reason for its introduction: innovation. In order to distinguish us from the (arguably superior, more fun and better presented) Affliction Warlock, the developers tried to diverge our methods. We both rely on dots, but where the Affliction Locks use Malefic Grasp to “supercharge” their Dots, we “explode” them.
This is of course a valid option, although I find my nose rankling at the thought of being a dot class that removes its dots willingly. Mind Spike at least was only marginally useful, so it was safe to ignore this spell all through Cataclysm until the Spine of Deathwing encounter. Shadow Word: Insanity is liable to become one of those spells the entire spec is going to be balanced around.
Maybe this is a gesture towards the Shadowpriests who clamored for maths and complexity, or maybe it’s an unintended complicated mechanism – this is its second incarnation, with a more clear description.
Personally, I think the spell has potential in the margins. With superior gear, it’s possible an advanced Dot-Explode-Dot cycle becomes a bursty and viable way of dealing with bosses. It’s also a good way to ensure that your dots will not go to waste, as any lost potential is pumped into its instant damage. With proper development it might have use, but it will require more information and a truckload of maths to figure out when it’s useful and when it’s a loss.