I think it's supposed to be crimstone, but my character doesn't know the password at all, even though she's been to Nasradin's Tomb through the catacombs. So when you go to Rosenborg, it should be simple to find the cave and open it. BTW, I'm playing Car-Arcanum, so maybe it works differently? Thanks!
Crimstone is the password to Nasrudin's tomb, and I have no idea how to discover that. Unofficial Patching time, perhaps? Also, I don't think Mannox's hole requires a password. Mannox will let you slide in without saying so much as a word.
The password is 'truth', you have to talk to the historian at the temple about both the remains AND the drawing in blood on the lid of the coffin.