Don't Fear Us Reapers

There's something about being at the top of the food chain, it makes you believe that you're invincible. That even though the lower creatures may get one or two of you, you can destroy hundreds of them. So what happens when you get knocked down from the top of the food chain?
You take drastic measures.
Some would consider merely advancing weapons development a drastic measure.
What do you do, when you, a human being, become the weapon being advanced?

In the future, an apocalypse will come, it will not be zombies, but monsters of myth and legend. Humanity was decimated as scientists worked to create something, anything to fight the threat.
The only thing that worked however, was a serum that created super soldiers, called Reapers. Little is known about the long term effects of the Reaper Serum. But the fate of humanity depends on their ability to fight and kill creatures even more powerful than themselves.