Slender is probably one of the most themes used for a horror map, where the maker tried to make it the most possible way like the original game, but never in the same exact way.
After having made a special mob called Shadow that chases the player, stopping only some meters before, some people have gave me the idea to recreate the original Slender in Vanilla Minecraft. My version of the monster can teleport randomly in 8 possible positions relative to the player and, most important, will start to kill the player only if he seen it for too long. This mob is a concept, because can be a little buggy due to some possible issues, but is a good start to create a fully complete version of Slender.

Hope you appreciate =)