Horde Command Pack," has been temporarily delayed. Epic Games made the announcement on its forums, blaming a "technical issue" for the last-minute postponement.
"We have folks working around the clock to resolve this issue ASAP so you can all play and enjoy the Command Pack," Epic noted. "The Epic Games Community team will stick around and keep you posted with any further information."
Epic informs us that the current delay will not affect the launch plans for the second piece of DLC, "RAAM's Shadow," already scheduled for December.
Update: Less than a day, good on Epic. The "Horde Command Pack" is available now.