Who is Magnus the Mad from The Floating Island and what is their importance?

Magnus the Mad, who never physically appears in the novel, was Ven's great-grandfather. He is described as a curious man, who wasn't content to live in the underworld. His travels to the Great Overworld took him to a seaside port town, Vaarn, where he became a shipbuilder. This was a scandal among his people because the Nain were not only known to live underground, but also known to dislike water.