The monster is the Grand Matriarch as the dev just said. She's the purple monster-looking face in the final picture (40_16.png) on the first post. And she will presumably be trying to shag the MC based on the plot as it stands at present.
Though that's just a theory. A game theory.
If Lament isn't giving more info than that it's either because they haven't decided exactly what it might entail yet (as it's still a ways off in the development cycle) or they just don't want to. Either are valid options. If you're worried about what that content might be, say no to the question in-game and skip it.
Otherwise, you can just wait and see like the rest of us.