When "Slender: The Eight Pages" first hit, it scared the hell out of a great many people. The gameplay was simple enough, the environment atmospheric, and the game's lack of price tag made it an indie horror darling. It showed audiences and developers alike that making a game scary wasn't an overly complicated process.
However, it was still a bit rough around the edges, which is what sets the stage for "The Arrival." Not considered a direct sequel, but it's more of a re-imagining; "The Eight Pages" with a bit more polish. As such, "Arrival's" gameplay is comparable to "Eight Pages," but it's been beefed up, loaded with a significantly improved visual aesthetic, and a fully fleshed single player campaign mode.
Other "proxies" under the Slender Man's influence join the anorexic adversary, ramping up action a smidge.
Same terrifiying gameplay, with a more complete experience.
If you're interested, you can grab "The Arrival" from the official website for $10 for PC or Mac. Console ports are under consideration, but they're dependent on the game's performance on the initial release platforms.