What if I have a level 55 character on one realm but want to create a death knight on another realm to play with friends?

Q. What if I have a level 55 character on one realm but want to create a death knight on another realm to play with friends?
A. At the launch of Wrath of the Lich King, you will only be able to create a death knight on the same realm where you have an existing level 55 or higher character. In the future, we plan to allow the creation of a death knight on any realm (provided you have at least one level 55+ character).