It's a poster's illustration for my roleplaying association's main event, the CIIL. Every year, we make a theme with a play of world with the word "CIIL". And this year, it's CIIL Alive (Still Alive). So it will be zombie and Portal themed ! Hence the zombie unicorn in Aperture (the unicorn is the emblem of my association).
Companion Cube, Turret, Space Core, Aperture Science and Portal©Valve