Yeah, it would be really awesome to add a creature with Slender's features! Normaly I like the option to kill everything (which I can't..), but this creature had to be unkillable/really hard to kill because he never enters combat and if it does it casts some kind of stun against everything inside a # foot radius and desapears to stalk you somewhere else.
Oh, and it would be cool if people/guards talked about beeing stalked by some weird creature/seeing frightening things and going crazy or something.
The stalking could start with a quest for people who don't want this feature that much or there could be an Illusion spell that would make the creature not stalk you for days or make the creature stalk someone else maybe..
The point is, I like the idea

Edit: I used the word 'stalk' a lot..