My biggest dark fetish is consensual beheadings and post-beheading severed head fucking.

It has always been difficult for me to find this particular combination, but I think I've had the best luck recently on pixiv.net. However, being a Japanese site it can be quite difficult to search even though the navigation and user-interface are all in English.

Pixiv has on the right-hand side of the screen (when viewing a single piece of work) a short list of three pictures similar to the one you're currently looking at, so when you find a picture you like you can reload that page a few times to quickly gather a small number of other pictures to view. This also chains, so when you're viewing each of those new pictures, on the right-hand side of the screen is three more. Veild616 and e50gun are a couple of my favorite artists there.

Gurochan has been a great place to get an eclectic mix of good content, but not necessarily consistent or complete when looking for particular things or collections. I do like it here a lot though and often find things here that I can't (or can't easily) find elsewhere.

I've also been on the Dark Fetish Network (DarkFetishNet.com) where you can find groups and forums dedicated to some particular fetish. Many of the members share there work there and nowhere else because the site is Facebook-like and people simply post to their profiles or their groups. Much of the content I've seen there I haven't seen here or elsewhere.

Finally, g.e-hentai.org is an amazing site for finding collections of pictures, even guro. I've found massive collections taken from hentai games, compilations from/of artists, and even read whole comic books and hentai manga. When you open a gallery and it warns you that the content is "offensive to everyone" it's a good sign that you've found one containing guro or some other dark fetish. This is only very occasionally wrong.