Youmu's Cthulhu Mythos is a web video series published by Asagiri Kafuka in Nico Nico Douga from January 2012 to February 2013. It has a total 32 chapters.
The series can be seen in:
It got a manga adaptation named Minase Youmu to Hontou wa Kowai Cthulhu Shinwa (水瀬陽夢と本当はこわいクトゥルフ神話), but with its names changed to avoid copyright, illustrated by Masahiko Yosi.