The rights to Marble Hornets or this song. If you guessed right, here's your prize: a nice, warm internet hug. This video is obviously heavily based on Marble Hornets; this definitely isn't the phoney baloney Masky made by...certain fans that's... *cough* Cheesecake shy dork *cough*
Tim is a great character all on his own, and his addiction to his medication is actually justified--it keeps his homicidal personality and "Slender Sickness" symptoms at bay. Therefore, the song is quite suiting. If you've never watched the Marble Hornets series, I'd highly recommend it over at the Marble Hornets channel.