So, I'm using Triamore in my campaign as a NPC covenant. In addtion, I'm going to run the excellent adventures Ghoul of St. Lazare and Festival of the Damned very soon. In my campaign, the town of Vezay, where the action in Ghoul and Festival takes place, is located on the opposite side of the Ardennes, across from Triamore. The body the magi finds in the river, washes down from Vezay, along the Meuse, to Triamore.
So, who rules the lands on the other side of the Ardennes? Is Vezay (which is fictional, from what I understand) part of France or the Holy Roman Empire? I'm asking, since a noble lord makes an appearance in Festival of the Damned. Who could this be?
South of the Ardennes you find what today is called Lorraine. In 1220 it is called the Duchy of Upper Lorraine, under the Duke of Lorraine.
Both Upper and Lower Lorraine are without reservations parts of the Holy Roman Empire since about 925.
So where does the border go between Upper and Lower Lorraine? Does it follow the forest at all? And do we know what type of nobleman who controls that particual area?
Around the Maas it should in 1220 roughly have followed today's border of France with its neighbours Belgium and Luxembourg: so most of the Ardennes were Lower Lorraine.
I can't give you any small nobility along the southern Ardennes - those you have to research on your own by reading up on local history. But the county of Verdun went to the bishop of Verdun in the 12th century - so this bishop is likely the local power your PCs have to look out for.