After returning to the guest mansion once the sky had darkened, Cian changed into a new set of clothes. He had been invited to the guest mansion of Megaris for a banquet dinner arranged by his sister, and only close ones were invited, including Prince Arlan and his retinue.
Cian had to ensure the herb remained safely concealed, as it was a banned substance and couldn't be discovered. Accompanied by Eliot, he made his way to the Megaris guest mansion, which was adjacent to his own. They were directed to the rear garden of the mansion, where the arrangements had been made.
The garden was adorned with numerous lamps, illuminating the area and highlighting the lovely blossoms growing within, creating a pathway leading to the banquet area.
Upon arriving, Cian found Arlan and his knights already present, along with Arlan's personal aide, Oriana. Seren, who had been engaged in conversation with other guests, hurried over to him upon spotting him.
What happened to Arlan, you can read in chapter 267 onwards from the 3rd book. Also, the opera house chapter is 261 from the 3rd book.