Iason Heine:
Well, I can't help with the Emperor's Key, but the Witness of Dawn... hmm.
The Lords of Dawn have long used that term for their first--their leader. The Leader, the Prima, is the first to welcome the rising sun on their special occasions, where he witnesses the passing of night into day.
The current Witness of Dawn is a man named Franz, but he doesn't often meet others. I suggest you go first to the Temple of Einhasad in Aden Town and meet with Priest Wood. He should be able to set a meeting up between the two of you.