You need to do this 2 more times to unlock: Once you locate him, you need to approach and disturb him (he will not kill you). He will be standing facing a wall but his location within the house is random. You're going to need to look for a scary looking guy called Charlie. After a few seconds, a burnt down house will appear on your left and this is where the achievement is obtained. Cross the bridge and follow the path round to the right. Switch that on and you'll see a makeshift bridge in front of you. Follow the path around to the right and down a hill where you will see another Generator. You may notice Slender on the hill in front of you but don't be alarmed. Press A to switch it on, continue down past a round wire coil, stopping to pick up Scrapbook 9/50 from on top of it, and follow the path around to the left. If you wish to get it now, you need to leave the house and take a diagonal path towards a generator.

I did it once on this playthrough, once on Hardcore, and the final time just to improve my time for the speed run. So, you can either do that now or wait until a later time. Once outside, there is one more achievement you can pick up on this stage but it requires you to restart the Prologue twice to obtain it. So, now you're done here and you can leave through the gate you previously unlocked.

Slenderman the arrival key