MARATHON (IC thread link)
In the far flung depths of space, the peacekeepers are fighting a desperate war with the Zolkri Hierarchy, and they are losing. Pushed to their limits, the Peacekeepers have retreated further and further, already allowing the Zolkri to start penetrating into the milky way, cut off from many potential allies due to their prior antagonistic attitudes. As such, they have started to seek aid. First in the form of any sort of artifacts that can swing the war in their favor, secondly in turning back upon the great metahuman population of the homeworld of humanity, Earth.
Desperately fearing the possibility of an invasion of Earth, the Peacekeepers have scoured the depths of space for an advantage, and stumbled across an odd set of signals coming off from a region of space known as the great attractor, named for the influence of it's gravity that draws in countless galaxies towards it. These signals are indicative of a mechanical, artificial construct, and the prospect of an artificial machine large enough to attract much of the observable universe towards it through sheer gravity has the Peacekeepers excited.
Perhaps this is one of the many relics left behind by the mysterious Masari, a now long dead species of hypersapients who had achieved a higher level of technological development than virtually anything ever seen anywhere else by human eyes. A tall, fair, and elflike people who had spread across the universe and brought order to a chaotic cosmos, who had fought in the long past war in heaven, beings whom even the gods bowed before, and to whom whole galaxies were but building blocks for children.
And a people who had just suddenly disappeared, declining somewhat for a ten thousand year period, an eyeblink in their billions year old civilization, and then completely vanishing, not leaving so much as a single corpse, their last messages warning of "The sins of our thoughts and the tide of silvered death" and urging species that they said "were uplifted in our image and molded as our children to be our reclaimers to once again bring peace to the stars in the primordial's name."
They spoke of a great and terrible weapon, they spoke of the MARATHON rings, great and powerful weapons like nothing the universe had ever seen before. And the upper echelons of the Peacekeepers believe they may have just found one of these superweapons. And they are eager to claim it for humanity and use it as a bargaining chip against aliens.
To this effect, they have selected an elite team from the peacekeepers and their earthbound allies the enforcers and handpicked a fleet of ships lead by the APCKSS Battlecruiser Spirit of Liberty.
They would set out to the edge of the milky way galaxy to prepare for the massive intergalactic jump towards the great attractor with a team of soldiers and scientists to analyze the relic. But as their ships descended to pick up Peacekeeper friendly earthlings, many uninvited guests have stowed away (the explanation for non-enforcers joining up) for many purposes, whether to prove the Peacekeeper's malevolence or to see the stars for themselves.
What they don't know, is that the Hierarchy is following them.
This is the opener to my mega-arc, starting off as harmless enough fun where we get to engage in alien blasting, but it will soon tie into a much deeper saga. Don't worry about that too much right now, just enjoy the ride.
This details the Hierarchy's units: http://www.comicvine.com/profile/yuri_omega/about-me/. Looking at the Halo wiki, Mass effect wiki, Crysis wiki, Lexicanum (for the dark eldar), Crysis wiki, Supreme commander wiki, and Wikitroid will tell you all about the other races in the Hierarchy as they're largely ripped off from there. Look for the covenant species (minus the elites), the Ceph, the Dark Eldar, the Batarians, the Seraphim, and Space pirates.
For the most part, the enemies are mooks, feel free to show off how awesome you are against them.