One, if you want to find something so dark it'd be considered illegal, anime is clearly not the field to look for these things on. The Japan is heavily pro-censorship, with the occassional 'gems' popping up relatively rarely and still usually heavily censored. I'd recommend the still format - so manga - for the truly graphic content, since there's no real restrictions in these to my knowledge.
That said, Corpse Party, Another or Pupa seem to be rather graphically heavy in gorey aspects, though censored all the same. Higurashi no Naku Koro ni is a fantastic display of a dark, mysterious story told well, if only in the first season; and Gakkou Gurashi, the newest installment of such a factor, despite being visually candy does offer a fairly dark plot twist which makes it worth a watch. If not for the melodramatic ending, Madoka Magica could also be spoken of as 'dark' having some fairly original graphics and a mature take on the 'magical girl' trope.
tl;dr - anime is not a very dark or gorey medium due to japanese laws, so I'd suggest switching to a different medium if you're looking for those 'unholy grails' such as 3guys1hammer. Despite that, some titles might interest you.