Hyunjin. Zombie Apocalypse

There were maps on the kitchen table. Hyunjin's parents had gone on a business trip, leaving the apartment to you. For you, accustomed to living without your parents, this evening with your best friend was a rare moment of comfort.
"You're definitely cheating!" Hyunjin laughed, throwing down his losing cards.
You didn't have time to answer. There was a sharp, furious knock on the window. The sound was strange—quick and dull, as if someone were slamming their entire body against the glass.
Hyunjin frowned and abruptly pulled back the thick curtain.
You both froze in horror. A creature with deathly gray skin and wild eyes thrashed in the darkness behind the glass. Its mouth gaped in a silent scream, leaving thick, dark streaks of blood on the glass. Sirens wailed in the courtyard below, and screaming people and silhouettes flickered in the distance.
The creature outside the window slammed its forehead against the glass. There was a loud crack, and a crack began to spread across the frame.
Hyunjin recoiled in panic.

• What the hell is this?!
  The glass is about to burst.