Herobrine will now begin to show up on your map. If you come within 4 blocks of him, he will disappear after about a half second. He has a tendency to stand 1 block behind you in dimly lit areas. He will always look directly at you. Along with his presence, you might find blank signs, stacks of cobblestone, and torches you didn’t place. He has only attacked me once, and that’s after I shot an arrow at him, which he responded to by warping right in front of me and then draining all of my health at once.
Other than that he appears to pose no threat, but he’s pretty creepy. Have fun!