Dilan was the person Xaldin was when Xaldin was real.

And you know I've played too much Kingdom Hearts when that makes perfect sense.

Once upon a time there was Dilan. Dilan's boss started studying hearts. As part of this study he sacrificed the heart of one of his apprentices to darkness. This apprentice had gone nutso and started calling himself Ansem. So it may have been Xehanort sacrificing his own heart to darkness because things get kind of confused. Anyway Xehanort-as-Ansem convinced the other apprentices to peer into the darkness and as they thought it was Ansem-Ansem's orders they did it. At which point they lost their hearts and became abominations that the universe has spent the last ten years actively attempting to disbelieve out of existing.