Artorius Vigo was a Nilfgaardian mage and the uncle of Fringilla Vigo. It was he who introduced her to the mysteries of magic.
In the Blood and Wine expansion[]
While he does not appears directly in the "Blood and Wine" expansion, it is revealed that "Uncle Artorius" had created a magical land for young Annarietta and Syanna which they could access by uttering an incantation while reading a magic book.