Minecraft, the phenomenally popular Java-based game, has been constantly updated by Notch since its release and infancy. The latest update, 1.8, will add a new enemy in the form of the "Endermen". Notch provides an explanation for adding the Endermen on his official blog. In short, he wanted to evoke fear when players see them, but he wanted to do so in a manner that wasn't fatiguing or frustrating. He also explains the development process for the Endermen, and other ideas he had for them.
The Endermen are extremely tall enemies (roughly three meters in height, according to Notch), who must not be looked at, under any circumstances. Notch explains that the main events in Minecraft happen when in an area near the character, and said that he wanted to change this a little. For example, creepers will only explode when near the character, and fires no longer spread indefinitely. The Endermen also have long limbs and the ability to move blocks around at will, potentially destroying or altering the landscape.
The Endermen are passive unless you look at them. If you look at them, then they will turn and stare at you. When this happens, you had better be ready for a fight, for the second that you stop looking at the Endermen they will move towards you. But they won't shuffle or move slowly - they are very quick creatures, aided further by the fact they can teleport. If they can't reach you in a reasonable amount of time then they will rapidly teleport, roughly once per second. Notch explains that he likes his latest creation because he knows that when it attacks, it is the player's fault it attacks. The player can stand looking at the Endermen until their strategy has formed, but as soon as they look away it's time to be prepared for combat.
The Minecraft Wiki contains only one image of what the Enderman looks like, and there is no guarantee that it is accurate, though it has been included: