Phalanx Covenant
The Phalanx Covenant was a summer crossover that ran through Marvel Comics' X-Books comic series (such as "Uncanny X-Men" and "X-Men") in the late 90s. It is also the name of the main group of the titular characters, the Phalanx.
Basic plot
The X-Men discover that a race of shape-shifting, assimilating techno-organic cyber beings have launched an attack on Earth, and have already began infiltrating humankind, and even the X-Men themselves.