To complete Osina’s request, go to the privy next to the combat arena that you visited during the Opus Magnum quest, then interact with the stalls to reveal the demons. Since it just looks like a loo to you, go back to Osina and he sends you off again to investigate a mysterious door on the Maiden side of the tower.

After you have completed the Divine Messenger Quest in Kingdom Come: Deliverance 2, you will be aware of what exactly is going on. Sooner, Otto Von Bergow will gather his army and reach Nabakov to defeat the bandits and,

You need to speak with three people about where the demons are coming from within Trosky Castle: Manyeta, Bertha, and the castle blacksmith. They can point you toward where these demons are appearing in Kingdom Come 2, and then you can sprinkle those locations with holy water, cleansing them.