Agor.io

When you open this, an screamer of Jeff The Killer Pops up with an ear-piercing scream

But wait, if you want to close it, its better you reset your PC/Mac or anything (less phones), because this pop-up with a message "U SCARED BRO LOL" so, be carefull, this also would make you get a seizure! don't mispell agar.io!