Last time, I covered the various defensive cooldowns DK tanks have at their disposal. This time, I'll be explaining how to make it so you don't need them in all but the hardest fights. I will also explain what is meant by 'uncrittable', 'uncrushable' and 'defense capped'.
Cooldowns are 'active mitigation'; they mitigate damage after you activate them, and can only be up part of the time. 'Passive mitigation', by contrast, is the defenses you have all the time, and do not need to be activated.
Death knights have 7 basic types of passive mitigation, they are: parry, dodge, miss chance, armor, resistance, damage reduction, and magic damage reduction.
Parry is increased by collecting gear with parry rating or defense rating on it, and thanks to our class' passive ability, Forceful Deflection, every 4 points of Strength on our gear grants us 1 parry rating. Parry causes melee attacks made from in front of the death knight to do no damage, and when it does, it makes our next melee attack come 40% sooner. This 'parry haste' effect can allow more casts of Rune Strike, but also applies to enemies; if they parry your or the DPS' attacks, it can increase their attack speed on you. When combined with Spell Deflection, a tier 4 blood talent, you can also parry spells (even from behind) to take only partial damage from them.
Dodge can be raised via dodge rating, defense rating, agility, and the tier 1 unholy talent Anticipation (which I recommend for all DK tanks.) Dodge, like parry, causes melee attacks made from the front to do no damage, but has no side effects. For two-handed weapon users, there are no good weapons with tanking stats on them, so agility, for the large amount of dodge it grants, is a great second choice for a tank weapon, (followed by Strength, Stamina and Expertise, not necessarily in that order.)
Miss chance comes from the frost talent Frigid Dreadplate, (for melee attacks only,) and defense skill, which comes from defense rating. Each point of defense skill beyond 400 grants .04% parry, dodge, and miss, (and reduced chance to be crit) for a total of .12% mitigation per point. Miss chance functions mostly the same as dodge, except it applies to spells, ranged attacks and attacks from behind. The Rune of the Stoneskin Gargoyle grants 25 defense skill, not rating, by the way.
The reduced chance to be crit component of defense converts hits that would have been crits back into regular hits (which can then be mitigated against normally by parry, dodge and miss chance.) Raid bosses have a 5.6% chance to crit, and thus 140 defense skill over the base 400 for a level 80 will prevent a raid boss from ever landing a crit (which is why tanks are called 'uncrittable'. This does not apply in PvP unless the opponent also only has a 5.6% chance to crit.) This is what is known as the 'defense cap', because additional defense beyond 540 skill will not reduce your chance to be crit any further, however, it will still increase your parry, dodge, miss chance, and the effect of your Icebound Fortitude. As a side note, the reduced chance to be crit from resilience stacks normally with that from defense, and can be used to make becoming uncrittable easier for beginning tanks. Note that for 5man heroics, only 535 defense is needed to be uncrittable.
Armor reduces the amount of incoming physical damage by a percentage, which can be found in your character sheet. The math of how armor converts to physical damage reduction is a bit complicated, and I do not fully understand it, however I can say that armor suffers strongly from diminishing returns, making it less and less effective per point, and caps at 75% reduction. Armor typically comes from the plate items you wear, but can occasionally be found on other items (note that the armor on non-plate items and armor from enchants does not get boosted by talents or buffs.) The tier 1 frost talent Toughness and Frost Presence both increase armor by a very large amount, and I strongly recommend both for all DK tanks.
Resistance, like armor is complicated, and unlike armor effects only magical damage of the correct school of magic (holy magic cannot be resisted.) Resistance reduces magic damage of the correct school by .18% per point, and caps at 75%, or 415 resistance. Unlike armor, resistance to a specific school is nearly worthless for a DK tank, and should not be collected on gear. The resistance granted by the frost talent Acclimation, however, applies to all schools (and, indeed, can apply to multiple schools at the same time) and grants 150 resistance, or 27% damage reduction, when fully stacked. The one caveat is that Acclimation will only function if you are constantly taking hits from that type of magic, and will not work if you are just taking a hit here and there.
The complicated part of resistance, is when a spell would apply a debuff, the resistance can convert itself into a resist chance, which works like miss chance. Assume that, on average, the resist chance will lead you to taking the advertised amount less damage (ignoring the debuff's damage, if any).
Death knights can also gain general damage reduction that applies to all incoming damage from Frost Presence, Improved Frost Presence, and Blade Barrier. This reduction is self-explanatory.
Likewise, Magic Suppression grants magic damage reduction. Also self-explanatory.
As promised, I am also going to explain what is meant by 'uncrushable'. When a player is attacked by a mob (including hunter pets, but not other class' minions) that is at least 4 levels higher than them, that mob has a chance to land a crushing blow on the player for 50% extra damage. Back in BC, 'uncrushable' meant that a tank had so much miss chance and parry and dodge that every crushing blow would simply miss, (or be parried/dodged,) and thus, they could not be hit by a crushing blow. In WotLK, this does not matter, because our raid bosses are treated as level 83, and are not high enough level relative to our tanks to land a crushing blow at all. Uncrushable is meaningless in WotLK. Now don't forget it.
Next post I will cover attack tables and stacking mechanics. Until then, may your gear scores go farther in keeping you alive.