It's something of a reframing of Higurashi. Lambda is connected to Takano, and Featherine to Hanyuu. It's like Higurashi was, on a higher meta level than we got to see in it, a game between witches. But also Higurashi is explicitly referenced in Umineko as being a book. It's all very vague. There are various ways to look at it, but the one I favor would be spoilers.