I have a single Muscovy egg under a broody who up to a few days ago I would say was developing normally (never had or incubated Muscovy before but the embryo was moving when candled). Then one day I didn't see it moving thought nothing of it maybe just it's position with the egg at the time. We are about a week away from 35 days due anywhere between jun 26 and the 28th. Today when I went to check on the hen, I saw the egg had turned dark.
Anyway my question is do you think the egg is viable and intacted but looks large but then again I've never incubated an egg this long. The egg had been rather severely cracked prior to incubation so never thought it would be viable but it did develop and I did seal the hair like cracks with clear nail polish
I have seen duck eggs look like this when they go rotten. Might be a quitter. If it smells like funk toss it before it explodes under your broody and makes a mess of things with the others you have under her.
looking at the second pic...it looks like a star type crack in the top right of the egg. Might have been pecked or stepped on. Once there is small crack the egg will have bacteria growth pretty quickly. You can try candling again to see if there is any air sack at the rounded end. If you don't see a beak moving I'm going to say it is no good.