Though all the top management of the UAC were dead, and
so were most of their personnel down to the janitors, the
corporation survived, now under strict government
supervision. The UAC still sought the secret to matter
apportation, and continued its experiments under vastly
increased safety measures.
The UAC's base was set up on one of the moons of
Jupiter, hoping that the increased distance would enhance
Earth's safety if something went wrong. Marines were
stationed at the base, ready for anything.

The Invasion
Soon after the UAC opened its first Gate, the minions of
Hell made their first attack. Suddenly, through the Gate
flowed spiked, fanged, dripping techno-terrors. Meat
machines flailed their armored limbs and slavered with
bloodlust, seeking soft bleeding manflesh to rend. But in
their seeking, they found only death. The United States
Space Marine Corps was prepared for such an event, and they
poured molten death into the hordes of Hell. More demons
massed, hoping to overwhelm the defenders by their endless
numbers. But mass alone was no match for the marines. Set
up in defensive positions around the gate, the marines were
able to slaughter the monsters by the hundreds, taking few
losses.
As suddenly as it had begun, the invasion ended. The
last flaming skull screamed through, was hit by twenty
simultaneous shotgun blasts, and the chamber was silent
once more, except for the dripping of blood. Hell had
failed.
The research went on, more boldly, and less cautiously.
All the marines received the Silver Star from a grateful
government, and the UAC made an enormous contribution to
the Veterans Fund. The defensive positions were
strengthened, and the marines watched closely for another
attempt, all their attention drawn inward toward the Gates.
They were looking in the wrong direction.

The Rain of Monsters
Hell knew more than one trick. Months after the Gate
incident, the yearly supply ship came ahead of time. On
radar, the ship looked far larger than usual. And it was
coming from the wrong direction. Strange, but not
inexplicable. The lax radar operators reported the ship's
approach, and personnel went out to the landing field to
meet it. But it never landed. Instead, it hovered over the
base, miles in the air. The men and women looked up at it
and saw that something was terribly wrong.
The ship could not have come from Earth. It was huge,
kilometers long, and was built of bone, steel, flesh,
corruption, and death. It was a bio-mechano-magical
construct from the depths of Hell, and it had come through
space for its vengeance. Enormous doors, large as football
fields, irised open and hideous demons poured out, plunging
to the ground and blanketing the entire base with their
throbbing, pulsing bodies. They were everywhere at once.
The marines' defenses, set up to prevent an attack from the
direction of the Gate, were worthless. The monsters poured
through the sewers, the air vents, the hallways,
everywhere, rampaging, corrupting, and feasting.
Once more, the surviving humans were left as zombified
brain-dead monstrosities. Existing only to kill and kill
and kill.

It's Up To You
Only one man escaped death or zombification. The marine
commander. You. You weren't at the base when the skies
opened and devastation poured from the stars. You were
miles away, enjoying a walk across the moon's rough-hewn
landscape. Then you heard a snortling gurgle behind you and
whirled to face one of them. The beings that still haunted
your nightmares. Your reflexes weren't dulled by your
experiences, and you pulled out your pistol and blew the
imp to gory shreds.
Hot-footing it back to the base, you saw it all and
realized what had happened in a flash. The demon ship still
floated above the infested base. Your boys -- the men you'd
trained to fight and kill and die as no fighting man had
ever been trained before -- were dead. You were not there
when it happened, to die with them.
Unlike the ancient Samurai, who chose to die with their
men, you ♥♥♥♥♥♥ your pistol. You were going to kill for
your men. And if you died trying, well, you were going
to die anyway, some day. Death at the fangs of demons
might be the very worst way to die, but if they did
manage to get you, Hell would know it had been in a
fight.

After Hell's catastrophic invasion of Earth, the United
States took steps to prevent such an invasion from
recurring. The old UAC corporation was refounded, under
completely new management (because the old trustees and
stockholders were all dead, this wasn't much of a problem),
and sent to research tools and technologies to prevent such
an incursion from happening ever again.
Though the invasion had been stopped, and the remaining
demons were gradually being exterminated by mopping-up
squads, it was clear that the powers of Hell remained
strong. While the Spider Mastermind and Baphomet seemed to
no longer threaten, who knew what else lay outside?
Waiting. Watching. Preparing.
The new UAC began working on Quantum Accelerator Devices,
intended to close interdimensional Gates at a distance and
so prevent future incursions forever. The project began
innocently enough. Naturally the scientists, in order to
learn how to close Gates, had to relearn Gate technology
first. This ability was rapidly regained. Perhaps too
rapidly.

The Terror
Soon, beings from outside had their dire attention drawn
to the new experiments, and then, one day, a Gate opened in
the heart of the research complex. Unnatural horrors from
the pit poured in, ravening for destruction. But the UAC
scientists had learned their trade. The Quantum Accelerator
Device performed perfectly in its maiden test -- the
invasion Gate was closed instantly and permanently when the
Accelerator flicked on. A cyberdemon, halfway through, was
snipped in two when the Gate closed. Earth would now be
safe from literal invasion by Hell. At least, once the
technology could be set up around the globe.
The next day, a ring of seven Gates opened throughout
the base, and a monstrous legion rampaged through. The
Quantum Accelerator began putting out the Gates at once,
and within an hour, six were closed. But the hellish army
was now too strong, too numerous. The marines fought like
mad dogs, but were finally pulled down by the enemies'
claws. The scientists, marines, and bureaucrats were all
slain or transformed into undead mankillers.

The Task
The Quantum Accelerator and its prototypes are deep
inside the ravaged complex. A demon Gatekeeper guards them
and mans the last Gate of Hell. The government, frantic
that the Quantum Accelerator will be destroyed or used in
some alien fashion upon us, has ordered all marines to the
site at once, regardless of their location.
You were on leave at the beach, only a few minutes from
the complex, when you got the word. You suited up, grabbed
a pistol, and raced your pickup truck to the complex. When
you arrived, flashes of light, howls, and chanting could be
heard from the interior. Corpses were scattered everywhere.
Obviously the Gatekeeper was doing something inside --
something that would soon reach some kind of awful climax.
You know that within an hour or two, an entire division
of marines will arrive to assault the base with full
artillery and air support. You also know that they will be
too late. Far too late. The airplanes will be plucked from
the sky by floating terrors, the cannons melted by diabolic
rockets and fireballs, the soldiers blasted to shreds as
they charge into the armored shell of the UAC buildings. In
an hour or two, the monstrosities inside will have finished
their awful task, and will be prepared, once more, to take
on the world.
It's up to you. You have to enter the complex and stop
the Gatekeeper. Alone.