Seems like he became a vampire with some sword of battle lust..... and why didn't the teleportation circle take the hat?? It's awfully suspicious that it didn't...
From Skeleton to Ghoul, what is next? Lich or Vampire? I suppose he's steadily returning to his original form, though will he need blood or other forms of sustenance?
Right there, the author showed the common sense of typical dungeon adventurer parties in why they ALL refused to take someone like Loris along with them.