Runeforging

Runeforging is a skill specific to death knights, involving the affixing of runes to the death knight's weapon. It requires a runeforge, which can be found in four places: The upper chamber of Acherus: The Ebon Hold in the Eastern Plaguelands, the Shadow Vault in northern Icecrown, the fortress structure at Mord'Rethar: The Death Gate in southeastern Icecrown, and Darion Mograine's runeforge at Light's Hammer in Icecrown Citadel. After a small quest chain, the Shadow Vault—inhabited by hostile Scourge—can be captured to serve as a questing hub for the Knights of the Ebon Blade, allowing ease of access to the Vault's runeforges.

The Engraving Process
To engrave a rune on your weapon(s), open your spellbook to the General tab and click on the Runeforging icon. You'll see a list of Runes that you know. Choose a rune, then click the Engrave button. Your mouse pointer turns blue—use it to click on the weapon you want engraved. It helps to already have your character profile or bags open.

Runes
As of Patch 3.3.0, there are ten different runes that can be affixed to your weapon. Four are for both one- and two-handed weapons, three are for one-handed weapons only, and three are for two-handed weapons only. The runes affixed to a weapon can be changed by additional runeforging, but weapons can have only one rune at a time.

Tips
Runes are always better than weapon enchantments. A death knight should never pay to get a weapon enchanted.

In general, the best runes are Fallen Crusader for DPS and Stoneskin Gargoyle/Nerubian Carapace for tanking. For dual-wield DPS, Razorice on main hand and Fallen Crusader on offhand provides the largest DPS increase.