So, let me see if I have this straight (spoilers)...
[[[ MAJOR SPOILERS AHEAD ]]]
So the bad guy is...
An ancient supernatural tunnel/cave that feeds on blood it soaks through the tunnel's floor and walls...
which sprouts tongue, tentacle and worm like appendages...
and can also transform part of itself from what looks like a rock formation into a giant, horny slug creature...
that needs to mate with human females...
plus it also releases some sort of poison into the body of water around the cave...
which transforms those who come into contact with the water into a primal creature with superhuman strength and speed, a need to hunt/eat often and an instinct to bring "food" to the tunnel.
Oh yeah, plus there are tiny flying swarm insects that can quickly eat through plastic and car tires like it's nothing.
Hmm, did I miss anything?