The Phalanx Covenant (1)

Rushing to investigate a disturbance downtown, the X-Men find Sabretooth on the rampage again, but realize too late that it isn't really Sabretooth at all. It is the Phalanx, a voracious alien life-form that can assume the guise of anything or anyone. Before long, the X-Men are all captured, except for Beast, who must find a way to stop the alien horror from devouring the entire Earth.