Runeforge

From WoWWiki

A runeforge is a special type of forge used by death knights to emblazon their weapon with runes. A runeforge is required to modify any runes already in your weapon (done via runeforging). Runeforges are found in Acherus: The Ebon Hold, the death knight starting area, which are set around the room inside the necropolis. Runeforges can also be found in Naxxramas and the Icecrown Citadel.