Ceph Homeworld is a barren and larger planet than the Earth that located in the Alpha Quadrant of the Milky Way galaxy at somewhere beyond the fringe of the galaxy.
Nothing is known about the planet except that is bigger that Earth, inhabited by a much more advanced Ceph and from the look of it, its resources are already exhausted. Whatever it is the reason why the Ceph on the planet decide to invade the Earth or not or if they have any connection with the overhive that sent the Ceph Warship from the M33 galaxy is unknown.
On the holographic map of the Milky Way galaxy at the cutscene of Mission 6 of Crysis 3. It was showed several location of all spiral arms of the galaxy are marked with cubic-shape crosshairs. On the galactic map at bottom-left us an image displays the galaxy filled with red dots, indicating that the Milky Way is already under the Ceph's control.
Even worse as there are locations in satellite galaxies that marked with crosshairs and there is a map showing the Milky Way's satellite galaxies map with all the dwarf galaxies marked with blue spheres orbiting around the known galaxy, the image heavily suggests the Ceph is also in control of all Milky Way's satellite galaxies.