I, due to the tiny amount of information we have to work with, could come up with nothing more than a theory I'm ~78% sure in.

Anyway, here goes:
Case, likely while being at that dinner, consumed something that affected his brain in such a way that it caused him to develop hallucinations he believed to be actual, real things. Furthermore, as far as I'm aware, the coroner never reported that at least the remnants of the handprints on Case's neck, or the cuts on his fingertips existed. This leads me to the conclusion that the cause for Case's death was life-like hallucinations induced by something he consumed at the dinner, which then caused Case to die of fright. This, however, would only be possible if Case had some sort of medical condition that caused cardiomyopathy, such as diabetes, coronary heart disease, or even something simple as high blood pressure. I assume that he did actually have cardiomyopathy, since he reportedly took vitamin supplements daily, but that might have nothing to do with it.