Slender: The Eight Pages is the game that made indie horror mainstream. Parsec Productions released it free in 2012: you are alone in a dark forest at night with a torch, looking for eight pages pinned to trees, buildings and fences. That is the entire game — and it is terrifying.
Slender Man follows you. He never runs; he simply appears, closer each time, and looking directly at him builds static that eventually ends the game. Every page you collect makes him more aggressive, so the game gets harder precisely as you get closer to finishing it.
You win — the game ends. Getting there is the challenge, because Slender Man's behaviour becomes more aggressive with every page you pick up.
No. There is no weapon and no way to defend yourself. Movement, the torch and not looking at him are the only tools you have.
Yes — Parsec Productions released it in 2012 and it became the game that popularised the Slender Man mythos and modern indie horror generally.