Adding to this theory

If the soverigns did get some kind of prophecy about Sunny, its possible that they might know some of his abilities. They might know about the multiple bodies and ability to copy things. Mordrets abilities are really similar to Sunnys in a lot of ways. Mordret can copy things (like shadow dance) and got multiple bodies (like sunnys saint ability). Mordret can even move around really quickly through mirrors (like shadow step). It would make a lot of sense if the soveriegns thought mordret was the one they were looking for.