Years ago, at the precipice of Human civilization, crystals of deep reds and blues began to protrude from the ground. With the crystals came strange, demon-like creatures. Their forms varied from humanoid figures, to large animals. One thing they all had in common was that they wore masks. Their appearance sent fear throughout humans, they holed themselves up inside their vast cities.

They were right to be afraid.

A short while after these demon-like creatures appeared, they began seeking out Human civilizations in large numbers. One by one, over the course of years they wiped out city after city. Humanity needed a way to fight back.

They had to do something.

Thus, they began to experiment on their kin in an attempt to create soldiers who could fight the creatures that threatened their existence. Years continued to pass, and cities continued to be destroyed as they worked.

Then, the demons reached the city where the experimentation was taking place. What the demons were met with was a small group of individuals who wore what looked like half a gas mask on the lower half of their face. The creatures stared at the small band, and then began to attack.

At first, the masked individuals did nothing, letting the demons run passed them into the city proper. There, the demons began to tear into people. Maiming them. As the scent of blood filled the air, the band of masked individuals began to react. The blood of the humans rose from the ground and surrounded them, beginning to form shapes. It only took a few seconds for all them to have individualized weapons.

With them, the soldiers began to attack.

As they took out demon after demon, they began to notice strange particles leave their enemies' bodies. One of them, ever curious, unclasped their masked. Their mouth opened wide, revealing long jagged fangs. Haphazardly, they grabbed one of the demons, and bit deep into it's throat. They drank deep of the demon's essence. So much so, that when they let go of the creature it collapsed into a pile of dust.

With their curiosity sated, they returned their mask to their face and turned back to the battle at hand. As they moved to attack one if the demons, they found themselves lighter, more agile, faster, and much stronger.

They also found that they could draw blood to themselves and use it for various types of attacks, including magic.

Three months have passed since their first battle. What was originally a group of six soldiers protecting one city split into three groups of two to travel their broken world in an effort to rid it of the demons that plagued Humanity.

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I'd love to get more input on this idea! If you're interested in playing this with me, send me a PM.