Deciding whether to tell Von Bergow about Olda Semine running around with bandits is one of the first proper choices you'll make in Kingdom Come: Deliverance 2, and it has a variety of consequences for the region and particularly the characters at Semine Castle.
You'll be presented with this choice during the Necessary Evil quest, where Von Bergow tasks you with interrogating a captured bandit.
If you do your job well, you'll discover that Olda Semine has been collaborating with the bandits trying to kill Von Bergow, which explains why he disappeared during his wedding, right around the time the lord of Trosky was ambushed on the road. Here I'll explain the outcomes of telling him and lying to him, as well as the consequences of the subsequent choice at Semine that telling him opens up. This guide will contain spoilers around the Necessary Evil quest.
Should you tell Von Bergow about Olda Semine?
Provided you discover that Olda Semine is involved with the bandits from the prisoner in the Necessary Evil quest, you'll be given the choice to either tell Von Bergow or keep it secret. Even though Hans asks you what to do after exiting the dungeon, the real choice is made when you speak to Von Bergow. Here's what happens in both cases.
Lying to Von Bergow
If you don't tell Von Bergow about Semine, he'll believe you, and you'll immediately skip to the next quest, "For Victory!", which is one of the final ones in the region. You'll also notice that you get the Silence is Golden achievement, stating that "Thanks to you the raid of Semine didn't happen". This is definitely the best outcome for the region, since Semine Castle won't be razed. However, if the raid does happen, there is an extra choice you'll have to make which I'll detail below.
Telling Von Bergow
If you tell Von Bergow about Olda Semine working with the bandits, he tasks Bergow Hashek with assembling a force to ride to Semine Castle with you and Hans accompanying. It's worth noting that if you fail to discover Olda Semine was involved with the bandits in the interrogation, Hashek will bring up Olda's disappearance at the wedding with Von Bergow instead, and the raid on Semine will still happen.
Along the way it becomes pretty clear that Hashek is still smarting from the ambush—where some of his friends were killed—and that he plans to take no prisoners. Once inside the keep you'll have a confrontation with lord Semine about his son. The lord doesn't want to hand over Olda, and no matter what you do, this will escalate into a fight. You can choose whether to side with Hashek and execute Olda, or side with Semine to give him a chance to explain himself.
Should you side with Hashek or Semine?
Siding with Hashek is definitely the most evil outcome here—it's clear that he plans to kill Olda and won't give him a fair trial. He also butchers the entire castle in pretty brutal fashion, including keeping Lord Semine alive so he can watch his son die. You'll find Olda in the attic, and even if you say you'll give him a fair trial, Hashek turns up and kills him before torching the castle.
If you side with Semine instead, you'll kill Hashek and his men. Olda Semine will appear and explain himself to you, and it becomes clear that the only option is for Semine to burn down the castle, go into hiding, and for you to lie to Von Bergow that Hashek died in the assault. When you return to Trosky, you can lie to Von Bergow without having to make any checks, but it's worth noting you can get a secret cutscene if you choose to tell Von Bergow you killed Hashek, where Henry is led to the gallows and hanged.
So, if you choose to tell Von Bergow about Semine, the castle will be razed whether you side with Hashek or lord Semine. Overall, the best outcome is finding out about Olda Semine and keeping it secret, or siding with Semine if it does happen, since the castle's inhabitants won't all get brutalized.
It's also worth noting that if you avoid the raid on Semine, you can also return to Jan Semine later and explain to him that you didn't rat his son out to Von Bergow, for which he's very grateful.
