With a limited amount of spoil:
About the death temple, there was a note about an improved potion of perception and all the ingredient to brew one. It may help if you raise your perception, but you may do it without with a little observation and abuse of the isometric view.
Craft the diary and the blood together to make the death bane skillbook. It removes the death knight immunity for 3 turn with a 3 turn cooldown. The skill book doesn't disappear after learning the skill so the whole party can get it.
