Death Knight - Unholy Tank

Purpose

Unholy Death Knight tanks excel at multi-mob tanking, better than any other Death Knight tank. They also generate threat well on single mob fights (bosses) by doing large quantities of damage as opposed to a warrior or paladin tank who have specific threat increased abilities.

Talents

The perfect DK tank spec is still debatable at the moment, but there are three specific talents to keep in EVERY build:
Blade Barrier∞ (in the blood tree)
Toughness∞ (in the frost tree)
Anticipation∞ (in the unholy tree)

Key talents to get for tanking:
Epidemic∞ - makes your diseases last longer, reducing the need to reapply
Scourge Strike∞ - The primary DPS (and thus TPS∞, or threat-per-second) strike for Unholy builds
Bone Shield∞ - The primary mitigation talent of Unholy tanking
Unholy Blight∞ - The signature talent of Unholy tanking

Key talents to avoid for tanking:
Summon Gargoyle∞ - nice for dps but does not contribute to your threat
Master of Ghouls∞ - nice for dps but does not contribute to your threat
Blood Caked Blade∞ - great for dps but since it counts as a physical swing it can be parried separately from your auto-attack, giving the boss double the chance to parry you (and gain up to 40% haste for his next attack)

Some sample builds for Unholy are (Taken from Elitist Jerks Forums∞):
Without Anti-Magic Zone∞
With Anti-Magic Zone∞

Without Lichborne∞ (author preference) - this build drops Lichborne in the Frost tree which frees up talents in the Blood and Unholy tree to generate more threat

Ability Rotation

While out of combat put down a Death and Decay∞ where you plan on tanking the mob(s). If there is a caster in the group make sure you Death Grip∞ that mob to you first (or Strangulate∞ so it runs to you)

Always open up with your diseases, Icy Touch∞ and Plague Strike∞. Follow this by using up your blood runes in the next two global cooldowns. This means if you're tanking a single target do 2 x Blood Strike∞ or for multiple targets 1 x Blood Strike and 1 x Pestilence∞ The objective is to keep Blade Barrier up 100% of the time, which is fairly easy to do, just make sure to use your blood runes when they are up. After your opening sequence your Frost and Unholy runes should be available for 2 x Scourge Strike. From this point, just rotate your Blood Strike (or Pestilence) and Scourge Strike, re-applying diseases when they are about to fall off. Always make sure your diseases are up!

There will come a point where you have a free global cooldown and extra runic power. Use this to cast either Death Coil (single mob) or Unholy Blight (multi-mob). It is the latter which makes Unholy Death Knights such powerful AoE tanks.

Key Cooldowns

Bone Shield∞ - this must be activated every cooldown (every 1 min) to reduce incoming damage to you
Icebound Fortitude∞ - use this in situations where you know a large damage spike is incoming or you have recently taken a large damage spike (to allow your healer to catch up)
Anti-Magic Shell∞ - use this in situations where you know a large magic damage spike is coming; this is very useful on dragons and bosses with long cast spells.

Horn of Winter∞ - not a cooldown per se, but it is important to keep it up since it not only increases your threat but also your dodge and parry

Stats (In Order of Importance)

Defense to uncrittable
Stamina
Strength
Dodge
Parry
Expertise to soft cap
Hit Rating to soft cap

Of course it is not possible to raise all of these stats at once. One of the best pieces of advice is to try to balance your stats. Don't focus too much on any particular stat (aside from uncrittable) or you will hurt your other stats too much. When you're starting to tank point for point dodge is worth more avoidance than parry, so given the choice choose dodge rating.

Once you are uncrittable, you want to try to get up over 30% avoidance with your dodge and parry combined, which is more than achievable with some crafted gear and quest rewards.

Recommended Glyphs

Major:
Glyph of Bone Shield∞ - adds an extra bone, thereby increasing the uptime of your Bone Shield, a must have
Glyph of Icebound Fortitude∞ - makes IBF cost no runic power, for those times when you're RP starved and absolutely must have it
Glyph of Scourge Strike∞ - applies diseases for you, saving global cooldowns, increasing threat
Glyph of Rune Strike∞ - increases your threat by a decent amount - this is a perfect glyph is you always have a Blessing of Sanctuary on

Minor:
Glyph of Pestilence∞ - a must for any tank, makes grabbing mobs much easier
Glyph of Horn of Winter∞ - saves on runic power, allowing you to use it on other cooldowns

Recommended Enchants

As with gear, choose an enchant which fills your greatest need. If you are not uncrittable then choose defense. If you are, choose stamina or avoidance instead.

Head - Arcanum of the Stalwart Protector∞ (+37 Stamina/+20 Defense Rating, Argent Crusade revered)
Shoulder - Greater Inscription of the Pinnacle∞ (+20 Dodge/+15 Defense Rating, Sons of Hodir Exalted) or Lesser Inscription of the Pinnacle∞ (+15 Dodge/+10 Defense Rating, Sons of Hodir Honored)
Cloak - Titanweave∞ (+16 Defense Rating)
Chest - Greater Defense∞ (+22 Defense Rating) or Powerful Stats∞ (+10 all stats) or Super Stats∞ (+8 all stats) or Super Health∞ (+275 health) - choose the enchant which fills your greatest need
Wrists - Major Defense∞ (+12 Defense Rating) or Expertise∞ (+15 Expertise Rating)
Gloves - Armsman∞ (+10 Parry Rating and 2% Threat) or Expertise∞ (+15 Expertise Rating) or Precision∞ (+20 Hit Rating)
Legs - Frosthide Leg Armor∞ (+55 Stamina/+22 Agility) or Jormungar Leg Armor∞ (+45 Stamina/+15 Agility)
Boots - Greater Fortitude∞ (+22 stamina) if you have Unholy Aura or Tuskarr's Vitality∞ (+15 Stamina/Minor Speed) if you do not
Weapon - Swordshattering∞(+4% parry) or Rune of the Stoneskin Gargoyle (only if under the defense cap). Please note that Swordshattering is worth more avoidance as long as you are uncrittable already.

If you can augment your own armor with tradeskills often those are better than the ones listed above. For example:
Enchanters - Enchant Ring - Stamina∞ (+24 Stamina to each ring)
Leatherworkers - Fur Lining - Stamina∞ (+90 Stamina to bracers)

Recommended Consumables

Flasks
Flask of Stoneblood or Lesser Flask of Toughness (only if not uncrittable through defense)

Battle Elixir
Elixir of Expertise or Elixir of Accuracy

Guardian Elixir
Elixir of Mighty Defense or Elixir of Protection

Food
Blackened Dragonfin

Additional Tips

Macros:
For the two strikes that you will be primarily using, Blood Strike and Scourge Strike, make a macro that casts Rune Strike before it every time:

Note: Rune Strike MUST come before Blood/Scourge Strike in the macro or else it won't work.

By using these two macros you will drastically increase your damage and threat. It is a common misconception that this will starve you of runic power, but since Rune Strike can only happen once every 3.X seconds (the speed of your weapon) this will almost never be the case. If there is a situation which is sensitive you keeping runic power (for spell interrupt for instance) you must manually activate Rune Strike, but this situation is not very often.

Buffs:
Pally buffs are preferred in the following order: Kings > Sanctuary > Might
If running an instance with a shaman ask them to put down a different earth totem than Strength of Earth, as it is the exact same buff as Horn of Winter (Stoneskin is a good choice).

Increasing Threat
As Death Knights our threat output is limited only by our Runes. We have two abilities to reset our runes, namely Blood Tap∞ and Empower Rune Weapon∞. Do not be afraid to use either ability on every cooldown, especially at the start of a fight when aggro is the most volatile.

Sources and Other Resources

DeathKnight.info forums∞
Elitist Jerks Forum - Death Knight Tanking Discussion∞
A post in the above Elitist Jerks forum that consolidates much of the information in the thread∞