Sja-anat says that "we were made and then unmade" and Shallan has the impression that the midnight mother is looking for something that she has lost. Their name also implies that they were once something and then changed. Ishar was one of Odium's most intractable, knowledgeable, and dangerous opponents. I have a hard time believing that he would do such a thing. We also have evidence of Unmade operating during the Heraldic epochs well before he broke.
Doing that kind of thing is like mistborn spoilers.