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Prior to patch 3.1, Unholy was the be all end all of Death Knight DPS specs (although the diseaseless Blood and die-hard dual-wielders tried to argue). There were a few significant changes made to the spec that altered the way it is played, but could hardly be considered "nerfs". Currently, Unholy DPS Death Knights do much the same thing as they always have, lots of Scourge Strike, and extensive use of a big, ugly, persistent Ghoul.
Unholy - 17/0/54
A crucial aspect of making this talent spec work, and therefore worth mentioning first and foremost, is that you must have the Glyph of Scourge Strike. Scourge Strike is the bread and butter of the Unholy DPS spec, and you want to use it as often as possible. Having the Glyph allows Scourge Strike to reapply diseases on its own, meaning that instead of hitting with a weak Plague Strike and weak Icy Touch, you can hit for a heavy Scourge Strike that has the same effect, more damage, and takes less time. You still want to open up with IT/PS, but from then on skip it and use Scourge Strike. After your diseases are up you need to immediately do two Blood Strikes to get your Death runes converting. Then, use all of your runes for Scourge Strikes until the Death runes reset to Blood Runes. Instead of putting up diseases again, go straight back into 2x Blood Strike, with Scourge Strikes. In the middle of the rotation, where convenient, you need to put up Unholy Blight. Only use Death Coil if you have Unholy Blight up and lots of extra Runic Power. Your Ghoul is a large part of your DPS, and should be active all of the time. If it dies, quickly resummon. Don't let anyone tell you to dismiss it, no matter what. If you're afraid of it pulling extra aggro in a situation, use a macro so that it only attacks the target you are currently attacking.
===Macro===
#show Tooltip
#show Scourge Strike
/petattack
/cast Scourge Strike
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Effective use of your Gargoyle is also crucial with this spec. Because of the long cooldown, make sure you know the duration of the fight and whether you will be able to use it multiple times or not. If not, use it during the time when you have the most personal buffs, like Heroism/Bloodlust, trinket procs, etc. Your Gargoyle's damage is based upon your stats at the moment you summon it, so get the most out of it!