HELP. Black spot with ring on duck egg

[FizzyIzzy7]

My cousin and I recently went to a lake a found a duck egg on the ground. Nearby we found the nest so we assume some kid might have been playing with it. We did not want to just leave it there so we brought it home which I know we should've done. We candled it and saw a embryo. I have hatched duck eggs befor so I know it was a embryo. Today I candled it and there was a dark spot on the egg I dont know what it is and I dont want to open it and find out. I have tryed searching it up and have found mixed results please help

 

[Orca5094]

It's possibly mold growing inside the egg. Did it have nice red blood vessels before when you saw an embryo? This egg is no longer alive, sorry to say.