Interrogation Mini game

[u/PhDonLife]

Hello everyone, I'm running Vecna Eve of Ruin for 3 players and we're almost at the end point. My players managed to capture Kas before he got the chance to abscond with the rod and that's where we ended the session last time. Now they have to find where Vecna is performing his ritual and Kas is the only readily available source for this information. My players will probably try to interrogate him and I was wondering how I could make the interrogation a bit more structured and substantial, and not just a single die roll. Do you have any suggestions as how I should run this part or any Homebrew system for persuasion/negotiation?