I'm sorry I ask this again, but even with searching and reading for hours I just didn't got a clue what I'm doing wrong.

I have two of this fancy golden symbol thingies. One from the ghost protectors of the woman whose bones I was carrying, on from the cell with the shadow.

I have, however, three scroll about these rituals, and a tome of some kind. Unlocked all shadow/magic barriers, too.

So now, where to get this last piece ? Do I have to perfome this ritual somehow ? How ? Or where the heck do I find the last piece ?

If I remember correctly, it is across the lettered floor - either on that last shadow creature or in the area directly behind where he was...

Oh, as said, I already got this piece of the stone.

In the north-west corner of the map, you'll find a statue. Talk to the statue, and he'll make you go through the rituals. Make sure you do your homework on the scrolls though, or he'll cook you. Once you complete the 3 rituals, you'll get another piece of the symbol.

Whatever you do, don't even talk to the dragon. He makes mince-meat outta your party.

Alternatively, you can kill the dragon and take his scales and Crom Faeyr part. But suit yourself.