Goodbye duvet cover. Even after swimming around in water and me trying to work with it in the water (I will keep the suggestion by @elbutterfield in mind for less extreme cases), it wouldn't untwist. I chopped it up. Inside were the following:
Two bath towels
Three pillowcases
Two hand towels
One dishtowel
Two dryer sheets
@publickovacs @elbutterfield
Lucky you. Could have been death defying