RUNEFORGING

Runeforge

This Death Knight only profession, called Runeforging, allows you to augment your rune blade with various runes that provide bonus effects. Modifying your rune blade requires a Rune Forge that are located in Ebon Hold and The Shadow Vault (in Icecrown). Rune forging will take the place of weapon enchants so you will have to decide between runeforging your weapon or getting an enchant. In general, the runeforges are superior to any enhcants you can get. Each specific runeforge also has restrictions to one or two handed weapons.

Rune of Cinderglacier

Rune of Cinderglacier should be used when you are in a damage dealing role and when your spec is either Frost or Unholy. This is because both specs do a lot of damage as magic, where as the most powerful attacks of Blood spec Death Knights are physical damage.

Rune of Razorice

If you are Frost spec and have a damage dealing role, you might want to consider using this. Frost is already the spec with the greatest single hits and if you like to see large numbers pop up on your screen this is the best choice - but only for Frost Death Knights that are in a damage dealing role.

Rune of Spellshattering/breaking

These two options will further compliment your role as a tank for bosses that do a lot of magic damage. What is important to remember is that if the boss only does a few magical attacks and the majority of his attacks are physical, this becomes a less attractive option. Save this one for bosses that use only magical attacks.

Rune of Lichbane

In order to get full use of this you will have to be fighting undead mobs, otherwise the other options are clearly superior. There are however many instances that are primarily undead such as Naxxramas.

Rune of Swordshattering/breaking

This is the clear choice for all Death Knights (regardless of spec) that wish to tank. Almost all mobs in the game deal the majority of their damage as physical attacks, which this will boost your avoidance against.

Rune of the Fallen Crusader

This option can be taken up by any damage dealing spec. However, unlike Unholy and Frost, this is the only option for Blood spec damage dealing Death Knights since the majority of damage being produced by this spec is physical.

Rune of the Stoneskin Gargoyle

Cinderglacier

If you are tanking and you still have not reached the defense cap then this is the clear choice for your weapon. There is no 1hand weapon version of this rune, as 1hand weapons already come with defense stats on them - unlike 2H weapons which do not.