The Phalanx is supposed to have its origin in the alien being Warlock's mortal remains, which were left on Doug (Cypher) Ramsey's grave. Someone took Warlock's ashes and began the experiment that became the Phalanx.
The target of this being was to turn the whole population of earth into technoorganic beings, killing what it couldn't absorb. At first, it didn't appear much, though it's possible to find it in some mutant issues, like when it tried to absorb Yukio. Later, when it decided the time had come, a great offensive was undertaken, kidnapping most members of the X-Men, some mutants from the next generation, and a whole town of humans to construct Babel's tower, in order to connect with other technoorganical entities from outer space.
It was thought vanquished by the combined effort of all the mutant teams, and two beings of the Phalanx : Douglocke, a combination of Warlock and Ramsey, and Lang, who didn't want to exterminate the whole population, but only the mutants. However, the Phalanx has now resurfaced and threatened the future of the third generation of mutants, but was defeated by Blink. To do this, she had to sacrifice her own life.
Most recently, the Phalanx have turned up in space and destroyed thousands of people in the Shi'Ar empire. It was stopped, in turn, by the combined forces of Bishop, Rogue, Gambit, Joseph (Magneto), Beast and his news caster friend, Trish Tilby, and, of all people, Deathbird.