Sern Underground

In order to more easily oppose Duke Prermu, a Serna by the name of Jyhennos, forms the Sern Underground. Over the next few years, he is able to establish it in all corners of the Barony but at the cost of his own life. --- Well, the overview of the general timeline, as concerns the resistance (Note that a lot of these do have specific dates but I haven't nailed them down so right now I'm noting them as date ranges):
~6 - 10 OI - When Baron Hertmeen wouldn't face him in battle, Duke Prermu split his forces at either end of the Zy Pass and slowly pressed in until the Baron was forced to meet him at the Battle of Schmich. Hertmeen was slain in battle at the hands of the Duke. The rest of Sern soon capitulated. ~16 - 20 OI - After several smaller battles, the Sern rebels were forced back and then destroyed at the Battle of Schny by Duke Prermu, leading to the occupation of the Ainkdt Weald. ~21 - 25 OI - In order to more easily oppose Duke Prermu, a Serna by the name of Jyhennos, forms the Sern Underground. Over the next few years, he is able to establish it in all corners of the Barony. ~26 - 30 OI - Duke Prermu, in preparation to siege the Plensch Tower, looks to purge the Sern Underground who might disrupt his supply lines. His purges, while brutal and bloody, are effective in driving the Underground into hiding.
After the Baron was slain, what remained of the army was driven out of Sern where it occupied the basically-autonomous-at-that-point Ainkdt Weald so it could regroup. I would actually think that this is where the groundwork of the Underground is laid because I believe there's communication between the forces in the Ainkdt Weald and Castellan of Eurkuppsch's forces. I'm not really sure who "Jyhennos", the official leader of the Underground but 1) I think that's a cover name (since it's basically just "John" in German) and 2) he probably served in Ainkdt Weald army during this time.
When I imagine this period of time, I think of this proto-Underground as specializing in communication and the movement of people (Getting soldiers from the other areas of Sern to the Ainkdt Weald to be trained for the upcoming uprising).
After 20 OI, with the actual army of Sern crushed twice, armed resistance wasn't really an option. Most of the military leaders had either died or turned coat due to lose of hope. On top of this, Sern is pretty war torn at this point due to two major crushed rebellions (+ a few minor ones) as their manpower is limited.
I actually think the Undergrounds objective starting off was actually to improve the morale of a very beaten down population, as such, I see them specializing in the moving and smuggling food to get it to people who need it. Was this necessarily illegal? Not really but considering heavy food taxes to supply the orcish war machine that was occupying the Ainkdt Weald and the major Strongholds and already preparing for another war, if you could skirt around these taxes, that means that you have more food to go around.
I think that it only later started to develop into a terrorist organization as the sons and daughters of the soldiers who had died in the first generation came of age and started to get involved. I'd actually probably put that at around ~25 OI.
At that point, I think you had a perfect recipe for radicalization: a personal blood vendetta, a golden age you don't remember but can idealize, a current oppression, and, due to Lussk culture and their views on who their family is, the willingness to die for your nation (because the Underground took care of the Sern, the Sern became the Underground's family).
So you start escalating from small terrorist acts to big ones until you've inconvenienced the Duke that he comes in with fire and sword and just cleans house because Prermu doesn't do anything by half measures.