Zayele

You shouldn't play Necroticblight on 1-2 targets ever. In any case, multistrike is better than mastery until 4-5+ targets.

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Unholy currently has a sizable lead over DW Frost and the gap is only going to grow next patch.

No, multistrike >= bonus armor >= crit/mastery is the stat priority in general. Haste is not as good as crit for DPS and is fairly poor for mitigation. There is no multistrike cap (aside from the 100% hard cap).

You should get 2pc owing to the fact that the chest and shoulders are well-itemized. As far as stats, you want strength > bonus armor >= multistrike/mastery/crit. Multistrike and crit are the two best DPS stats (multistrike being better than any stat aside from strength) while mastery is a survivability stat.

BoS is big burst, but definitely not for opening, for any spec. In order to maintain long BoS, you need to prepare for it: have high RP, enough runes to get more RP, and optimally ERW at the ready. UH is even less suited for opening with a high burst, because Big Timmy is a great part of it and it takes time to prepare (3-5 Death Coils). But other than that, you can easily roflstomp and Cata 25HC with any spec and if you are at 700+, also most MoP raids. Two specs use BoS predominantly - Blood and Unholy. In old content specifically, you will almost always be able to start encounters

1) For Frost, yes, Pebble does more damage than DC on a single target. 2) Whether or not you have a Vial is irrelevant; BoS will always outperform NP on a single target as Unholy. And assuming you play it well and have 4pc/legendary ring, BoS Unholy will do more damage than Frost on a single target.

Unholy has the highest opening burst of any DK spec; you take Breath of Sindragosa, not Defile or Necrotic Plague - the latter two are not burst talents at all. Of course BoS is not usable as DW Frost, but 2H Frost makes good use of it especially if you lack tier bonuses and such. DW Frost does not have very much opening burst, and it cannot take advantage of our strongest burst talent. This said, Cataclysm 25H content is totally trivial on any class, so it shouldn't matter too much.

Your recommendation for DPS trinkets is incomplete. The 4 trinkets to use are Discordant Chorus, Empty Drinking Horn, Unending Hunger, and Vial of Convulsive Shadows. Vial of Convulsive Shadows is absolutely competitive with EDH/Unending Hunger and beats them with a suitable fight length. Moreover, I have to question what this "3000 multistrike" threshold refers to. It certainly shouldn't refer to Defile; that much multistrike is indicative of a high DPS build, and such builds should talent Breath of Sindragosa. If this is with Breath of Sindragosa in mind, it is incorrect, because multis

Breath of Sindragosa does not stay up indefinitely in 6.2; do not expect much more than 30% uptime on longer fights. It still is the best talent on every fight, but it doesn't do as much damage as you noted. Still, 10-20 seconds per Breath sounds very low; are you pooling runic power prior to using it? Are you also pooling Blood Charges to minimize RP overcapping? Are you trying to AMS soak as much as possible (this is very easy on Xhul'horac)? Are you using Vial of Convulsive Shadows? It's important to note that BoS doesn't need to stay up for very long in order to beat the other ta