It isn't a large planet, and it isn't even visited more than a few times a cycle. A thousand ships from Earth must have passed through this system, but the forth measly planet from the system's sun has gone undisturbed for centuries.
That ends now.
For the Yautja, it's a crucial location; a valuable mineral is located there and there alone. Light as a feather, but harder than steel. The majority of Yautja weaponry is forged from this metal, and they aren't about to go back to using the crap from the Homeworld.
The base had been established for centuries, and hadnÃ¢â‚¬â„¢t grown much since its founding. It was not a glorious place to work, but it was a starting point. Little honor would come from time spent there, but simply living in such harsh conditions could toughen you up, and maintaining the place was an easy way to gain technical knowledge.
All of these were ways for younglings to get their feet in the door, and for older, disgraced warriors to begin the long path to regaining their honor. For a few, it was the only home they had.
So despite the hot days, despite the freezing, mist-covered nights, and the tree monsters, and the unstable terrain, and having to wait months for word from home, and even the eternally drunk groundskeeper, the Yautja stationed there had good reason to defend it should anything ever happen.
A contingent of Colonial Marines had been assigned to carry a shipment of Xenomorphs to a station in the Outer Rim. The ship was a re-purposed Conestoga model; tough and reliable, made in the good ole' days.
Even the bugs couldn't break out of the acid-proof containers they were sealed into, and even if they did, the computer system would detect it and promptly put a large metal spike through the head of their Queen.
They maintained a strict schedule; one squad awake to guard the bugs, the rest in hypersleep. They changed every month. It was said that there was no chance of escape, even by the bugs.
And it was true, which is why Weyland-Yutani, in cooperation with the government, planted a bomb in the engine room. When the computer (specially reprogrammed in secret) detected any strange signals, it automatically kill the queen and release the Xenomorphs.
The ship would crash on a Pred-controlled world (the only one they would dare pull shit like this) and chaos would ensue. Any survivors would promptly begin killing each other, and whatever was left could be "cleaned up" at a later date. WY and the government would get Predator tech, the mineral, and an "unprovoked attack by hostile extraterrestrials" to scare Earth into buying more weapons.
As we speak, the ship is moving into position...
The BattleField Map:
The planet is desert-like, but large portions of it are "shifting ground" under which the mineral is buried in its liquid form. This causes otherwise solid ground to slide around and generally make a mess of things.
The only life-forms (besides the obvious) are huge tree-like organisms that are inhabited by smaller, sucker-mouthed creatures. The tree-creatures are formidable, but their numbers are dwindling. Their skeletons form forests of bone-like spires. There's only one left that's any sort of threat to the Preds, and it's far away from the base. (Imagine a 30-foot tree that's bare of leaves and can move it's branches around)
The Xeno numbers are low, so they retreat rather than fight it out. They'll have to decide what to do without a Queen.
The idea here is to pull all 3 species out of their comfort zones. The Preds, for once, aren't ready for a fight and the marines have got to pull together what little weapons and equipment survived the crash, and find a way off the planet (by hijacking a Predator ship)
Predator objective - Kill everyone else and protect the mining outposts.
Aliens: Begin building a hive; kill everyone else.
Marines: Get off the planet by hijacking a Predator ship.