yea as far as i konw, demi etc wont kill you if you have 1hp. the bit about the tomb of the unkonwn king, i'v never seen before. but it sounds as if you've not started the mission properly or something i'd go back to see caraway and also as far as i konw casting demi on diablos don't do a thing, except maybe heal him!
thats what i think, but i'd wait for somone whos not mad to tell you somthing coherent!