Shadow-X (also known as the Dark X-Men) is a Marvel Comics supervillain group that only appeared in New Excalibur. All of their costumes are the colors of black and red.
They are not officially given a name until New Excalibur #19 where they are referred to as Shadow-X in the "Previously..." section and by Juggernaut on the final page of the issue. How this name came about is unexplained, and prior to this the team was called the Dark X-Men by fans, reviews, and even the Marvel preview descriptions. It refers to the fact that the team was created and controlled by the Shadow King - the Shadow King's "Shadow X"-Men.
Read more about Shadow-X: Fictional Team Biography, Notes