Just as a sidenote - there is a bloodstone the eyesocket of the big statue behind Cassandra's grave. And yes, I did have to look that up in a guide.
As for the spell to combine with the blood to get the death knight bane, it is somewhere in the Luculla Mines. As for getting to Cassandra's grave, you'll need to get through a door that doesn't look like a door.....it is a deception....
