Hmm... I don't really like Demons. The problem is, besides the fact that it's too close to Doom, is that it makes it a bit *too* surreal. How can we possibly have a believable plot with supernatural entities, and also having those entities in possession of technology, and having those entities mortal. Anyways, I came up with something which provides a bit of background information to the story, and explains just a bit about the "aliens". My overall story now is to have the aliens attacking the Moon, but the entire Moon thing was just a distraction for you while they were busy obliterating Earth. Doom 1 would deal with everything related to the Moon invasion, and then you come back and realize that the Moon was simply a decoy so they could take over Earth, at which point it goes into Doom II. Got it? Oh, and I must stress, the aliens are extremely barbaric, vicious enough to be practically Hellspawn, so I hope that would satisfy the demon crowd. Oh, the name I pulled out of my a$$, put whatever alien-sounding-name you feel fits in there if you don't like it.
They call themselves the Kier S'he Nell. They are a barbaric race, a race that lives for the thrill of slaughter and bloodshed. They have caused much pain and suffering throughout the galaxy, with their vast invasion forces. They have developed the ability to possess the minds of their enemies, and often their armies consist more of their opponents than of their own kind. They operate as a hive, with one horrible overlord controlling their actions, and if not for this fact, they would have destroyed themselves long ago. Despite their thirst for blood, they are extremely technologically advanced. In addition to their abilities to possess, they have developed a technology allowing them to travel across the universe at will, through teleportation portals. With two portals linked together, one can travel from one to the other in a matter of seconds. Ages ago, the Kier S'he Nell sent out scores of pods, each containing a portal, so as to secure their hold of the galaxy, and conquer all who opposed them. One such pod, destined for Earth, instead crashed into the Moon, and became buried deep with it's surface, unusable yet intact. It lay there for many hundreds of years, until it was uncovered by a mining operation on the Moon. Naturally, as soon as the alien artifact was discovered, the military was called in to investigate. The Kier S'he Nell, seeing that the time was right, attacked. Quickly, the mining colony and the military base were subdued, before there was even a chance to send out a distress signal. Having taken the base, the aliens turned their sights towards Earth. They had already sent another pod down, allowing them to attack Earth directly, when a single marine showed up. Due to his strong will, he could not be possessed. Of course, this wasn't too distressing to the Kier S'he Nell; for it meant that they would have to kill him, instead. But first, they had to make sure he didn't escape. A small group of recently possessed was ordered to destroy his ship. Then, it was time for the fun to begin.
"They put Doom maps in cereal?" -Duke Nukem
"DON'T CUDDLE THE MISSION!!!"
People who use correct grammar and punctuation are actually AndrewB trying to pretend he is a woman named Nanami.