I think the grim reaper is symbolic and means Zoro is really on verge of death , i don't think it is a real enemy. that medicine might be some product of a DF itself or so, so Zoro has to tame it like he tamed Enma , so if he defeats it he will survive if he could not so he will die. but he has CoC so he can defeat it. or could it be something related to why his left eye is closed?