The 4 beings inside 4 Black Chests are actually the 4 heroes that defeated the Shadow Queen 1,000 years ago. They became Black Chests when the Shadow Queen used the last bit of magic she had to curse four of the Crystal Stars and when one of the four heroes hid one of the Crystal Stars, they turned into Black Chests. The fact that they curse Mario and seem to have become "evil" in general may be a result of anger created by the suffering they endured in the chests.