Some spoilers:
Cassandra's bones: You don't have to place them on an altar or anything. Just use firespells on them once you dig them out.
The spell rests in the mines' temple, inside the big blue bubble. Have you been there?
You'll have to go through several mirrors. One mirror in particular gives you an option to travel somewhere, an option the other mirrors lack.
Well crap I don't even remember where that is. Was there a portal in it, if so what was it called?
EDIT: You're going to have to be a lot more obvious as to where those bones are. My Ranger had a 12 perception and I've searched the walls inside and outside. Nothing.
