After I left the castle, I looked at the warehouse that Prince Toni told me. If it's as he guessed, then the real King Harold is being imprisoned there. And that warehouse should be where a merchant stored his valuable collection.
Said merchant was supposed to be well-known for his love of magic tools. Hearing that, I made a plan to cause a trouble by stealing from that warehouse while disguising myself as 'Albertus'. If the real king is locked there, the cult members in this city, including the fake king himself will be in trouble once I sneaked in. I need to make sure that 'Albertus' will only leave a trace of himself stealing some magic tools, while erasing all traces of me looking for King Harold.
When I leave Prince Toni, he said he wanted to exchange a message with Fabio. So, Sonia is staying behind. As long as he wasn't caught calling Sonia, I think it will be fine.