A dead embryo will rot if it is large enough and dies early on. Unlike in a mammal body, where a dead embryo can be reabsorbed if not too big, a egg is a closed developmental system and wastes from dying cells can't be excreted. Which is why a double yolk eggs aren't recommended for setting.
BTW, if the one died, did you ever open it to see how far along they got?
BTW, if the one died, did you ever open it to see how far along they got?