You consider your options. There's no telling who was in the invisible building, or what they would do if they found out you were snooping around. And considering how deep into the park you were, there was little chance you could escape without this... thing, -- whatever it was -- catching you, and that's assuming this thing was confined to the park.
But how do you fight something you can't see?
There's no time to think about that, you clench a fist and let it fly. You hit something, it's warm and furry, but also tough, like a brick wall. Your fist emanates a searing pain as the beaker shards embed themselves into your knuckles. You let out a grunt of pain and withdraw your arm. But the pain wasn't a futile effort, you can hear the invisible beast howl in pain, maybe some of the glass shards stayed on him.
The feeling of triumph doesn't last long. Before you know it there's a blunt force hitting you in the stomach, and the next thing you know you're flying backwards. You land in the grass at least ten yards away from where you were a moment ago. You try to get to your feet, but you're struggling just to breathe. Whatever thing hit you with, it knocked the wind out of you.
Between forced, raspy breaths you can hear heavy footsteps approaching, they stop about a foot away from you on the right. Something sharp and damp clamps down on your right shoulder.
It hurts. But only for a second. BLEARG. Whatever the invisible monster is, it sounds disgusted, you hear the footsteps again, but this time they're running away from you. You turn to look at your shoulder, your shirt is dyed yellow-green from the chemical earlier, as well as ripped and damp from the invisible monster, but you don't appear to be bleeding.
You relax for a moment and catch your breath, then you sit up. It's getting too dark to see, if you don't leave soon-
THUNK
Before you can complete your thought a metallic sound rings out as your head begins to hurt and you pass out.
You don't know how much time passes before you wake up, but when you do...