The show makes many references to existing works, so I'm hoping it's also the case here. It's mentioned in Episode 2, around 21:30 mark. I grabbed two relevant screenshots.
The audio is as follows: "That manga... It's about a slave girl who's taken in as the Overlord's daughter and falls in love with him... She faces hardship after hardship, and when she and the Overlord finally come to understand each other..."