This poor caterpillar was parasitized by a wasp, probably a Braconid species. When I first saw it about 10 days earlier, it was sitting on this mound of wasp coccoons. When I took this photo it had moved off the mound and was facing it. The wasps have largely vacated the cocoons now. Unicorn prominents seem prone to this sort of thing even more than other caterpillars, though parasitism is common on moth caterpillars of all species.