ROM is not a spider. This is because of inaccurate translation + ambiguity of story telling. She is an arachnid, but if you look at her and her offspring she’s very much so a type of Parasitiformes, which include ticks and lice. This makes more sense than a spider, because Parasitiformes are known for drinking the blood of hosts, and blood consumption is a theme of gaining the mutations in the setting. Hard ticks are an example of parasitiform that have a hard head, and soft body, which are also properties of Rom.

I imagine that this ticks or mite lived within the college on the heads of the students and possibly Willem himself, and drank their blood, causing the mutation to be of huge size and all the other benefits of drinking blood in the setting.