I found it now! it is not in empire lands, and it is not in northen avernum. It is at central avernum, down the waterfall warren, north of the castle, east of the unfinished fort.
hum... apparently I have never gone there, it is not in my map. It is close to Ornotha Ziggurath, right? South of Garzadh's fortress, north of Erika's Tower, West of the Spire
I already got the one at Angierach and the one the extreme north of Vahnatai lands, the one you get help from a village. How can I find the last one? I'm totally clueless