Xover: Chronicles of the outer colonies

Halo/Mass Effect/Killzone: The Outer colonies were the first to suffer from the Covenant crusade, hundreds of planets burned and billions died, but some founded save havens in remote part of the UNSC space. Both Vekta and Helghan builded their own small empires and when they, together with Earth, find giant artifacts of unknown origin, a whole new world opens for them.