[So, apparently I'm a minor GM. Since people were bored, I rolled stuff.]
Laythe organizes her Cavalry to start sweeping the city in search of Gavin, instructing them to stay in large groups whilst doing so. Gavin is to be taken alive preferably, but killing him on the spot is acceptable.
[1] Your men fail to find Gavin. However, the population is alerted by their presence that something is going on. Some of the brightest citizens are wondering whose troops these men are, and a few start talking about the Lord letting foreign forces inside the city without talking about it to anyone. Of course, they are not pleased. [-1 to rolls for the population's reaction.]
Eurika is given a command to tell the people that an assassin, using black magic, made an attempt on William's life, which he survives, soon after William leaves. She is also to say that William has gone after the Assassin. The Manor is to be blocked up in an attempt to control the fog, once everyone is safely out.
Do what William told her to do (like a good gi- employee) - and also head out in her usual garb and ask around for a certain preacher.
[2-1+1] The citizens are not satisfied with Eurika's explanation at all. A taloist priest starts shouting about heresy in the Lord's Manor, and within a few minutes a riot is starting. Half the mob appears to be convinced that William caused the fog using black magic, and the other half are only there because there is probably good loot inside the building. [5] Despite the fog and the angry citizens, Eurika manages to get back into the manor without suffering any harm, but now she cannot leave. William's guards are trying to control the crowd,[3] and they manage to stop the mob from getting inside, but they don't have the necessary numbers to make them scatter. However, [4] some of Laythe's knights quickly learn about the fighting and start rushing towards the manor. Only a few of them will be needed to chase the mob, thankfully. As soon as they will get here, the angry peasants will no doubt return to their homes.
Unless Eurika finds a way to leave the manor despite the angry mob, she will be blocked inside for one turn. Since only a few knights are needed, Laythe can still use her cavalry.William sneaks out of the Manor wearing a thick mask, a spare set of armor, and a spare blade. He heads towards where the Old Lord is staying. Then he does a quick scout of the place, trying not to arouse suspicion.
[5] You find all the need equipment and you sneak out of the manor without a problem, before the crowd starts getting angry. It takes you only a few minutes to reach your destination. The Lord's Manor is [1] very heavily guarded by multiple mercenaries. There are guardsmen around the building and even a few crossbowmen at the windows. Any attempt to make a frontal assault would result in an unpleasant but very quick death. However, [4] you spot a window on the first floor that is not guarded by anyone. You could try entering using that way. [4] You think it leads to the kitchens, which are near the Lord's study. [2] You cannot see a thing of what's going on inside, so it's impossible to know if there are servants in the room.
Remaining hidden, organise a den in the poor section. Start preaching in secret, calling the recent string of bizarre events a sign of things to come, especially against the sinful and oppressive authorites.
Procure some religious texts of Taloisism, either by asking those who come to sermons or by proxy purchase. Try to form a a secret sect under the name of "Golden Brotherhood".
Keep collecting money from the preaching if possible.
You manage to remain hidden without a problem - no patrol ever made it near your den. There's alot of agitation in the city, and you can use that to remain undercover, for now. [3+1] You start preaching about the sins of the Lord and his crimes against Polca, and you are generally well received - mainly because the populace is unhappy with some of the Lord's recent actions, and the events in the manor prove that something magical (and, by definition, heretical) is going on in that building. You manage to find [2] no religious texts - no one is selling any, and the priests don't think you are a real preacher at all. [3-1] Because you do not have any texts, you fail to create your secret sect. [1] And apparently, people are willing to listen to your preaching, but not to give any donations for it. When you suggest that idea, most of your followers abandon you, viewing you as a simple charlatan looking for easy money.