Ok, this WILL BE A SPOILER, read it as you like i will put in spoiler clams and all:
I will under here write the hints and paths to success of solving the st.mannox mystery:
WARNING SPOILERS MAY OCCUR!
First in the tomb under the first panari temple where nasrudin is supposed to lie, if you look at the thing nasrudin is supposed to lie in (i dont actually know the name though lol) the stone should say something strange and impossible to understand written in blood+Gives you a sketch of a broken ring
This can be translated though, go to gunther, he will translate 2 words "opposite" and "truth".
For now we will focus on the broken ring, where can you find a broken ring? roseborough!
Go to the ring of brodgar, the stone here will tell you to go mostly east (a little bit south maybe but mostly east)
Remember now, you were told to go "opposite" head along the coast westwards now from roseborough.
You should a wheel of metall, in which you'll require a password, the password is a bit self-explanatory, it can only be the 2nd word "truth".
Innside you'll find st.mannox' possessions, returning and telling hardian about this will "solve the mystery of st mannox", this isn't a official quest though, but by doing so hadrian will allow you to take the glass key. (+2 other items, which are cool to)
Now all you need to do is go to the iron clan and use this guy, easy schmesy solved and solved.
EVEN BIGGER SPOILER: (Warning) do not do this if you're allied with the dark elves, something really bad will happend.
PS: if you're a dark elf ally there's allways the option to just kill hadrian, unlocking the seal on his items :)