Originally Posted by Apokx

A 55 on any server unlocks the death knight.
One DK per server(you could delete one and make a new one, but you cant have more than one per server).
When you create one, he starts at 55 in a special area just for DKs. You do the quests that explain the lore, give you the talents etc. Once you finish those, the area becomes "live" and you are allowed to leave and play the character as normal(with talents etc).

They also give you a full set of really nice blue quested gear, and you should be level 58, allowing you to go directly to Outlands.