How do I create a Death knight?

Currently, no quest line is required to create a death knight. Once you have bought and installed Wrath of the Lich King, all you need is one character on your -entire account- that is level 55 or higher. Once you have a character of that level, the Death knight class will be 'unlocked', and you will be able to create new characters using that class.

According to current information, players are allowed to have as many death knights per account as they desire, but they are restricted to -one per server/realm-. You can have one death knight on every server if you want, but no more than one on each server.

If you wish to create a death knight on a pvp or rp-pvp server, you cannot create the character on the opposite faction of what you currently play. For example, if you have alliance characters on a pvp or rp-pvp server, you cannot create a horde death knight on that same server.