The CW just posted the new trailer for the upcoming Supernatural episode that features H.P. Lovecraft and Cthulhu (in what form remains to be seen). Here it is:
Earlier we found out some info that may have changed somewhat by now:
In the upcoming two-part season finale of Supernatural on the CW, HP Lovecraft is going to make an appearance in the double episode, which is called "The Haunter of the Dark." Lovecraft fans will note the similarity in title to Lovecraft's own story "The Haunter of the Dark." Hmmmm...(via Holy_Trenchcoat)
Actor Peter Ciuffa is going to play HP Lovecraft himself, although it's not clear if this is going to be the "real" HP or just a character with the name. You may have seen Ciuffa (or you may not) in a few other sci-fi TV bit parts he's had, like playing an "angry parishoner" in V and a "driver" in Fringe.
The story "The Haunter of the Dark" is about a cult (surprise) that uses a "crazily angled stone" (surprise) called the Shining Trapezohedron that summons a terrible cosmic entity (surprise). The entity has forbidden knowledge (surprise) that humans can learn only at a terrible cost (surprise). It naturally enough takes place in Providence, Rhode Island (surprise).