Who is the evil wizard in 'The Castle in the Attic'?

Alastor is the evil wizard in "The Castle in the Attic."

Specifically, he is a character in a book published in 1985. The book's plot revolves around the adventures of a ten-year-old boy being able to shrink to the size of his friend, a toy Silver Knight in a toy castle gifted by an English nanny. The book's author is Elizabeth Winthrop (b. 1948). Winthrop is the great great niece of 26th U.S. President Theodore Roosevelt (October 27, 1858 - January 6, 1919).