There is no public footage of an actual Cthulhu Mythos ritual anywhere on the regular web. As you may have guessed, the Dark Web is an entirely different matter (Muahahaha!).
Dutch ALternative Rock band De Staat have produced a video for their song Witch Doctor which comes pretty close to picturing, what such a ritual might look like.
Actually, especially the end would go nicely with a summoning of Yog Sothoth, Nyarlathotep or Shub-Niggurath. Just take a look and enjoy, it is one great song and a phantastic video: