Alzur

Alzur was a famous mage.

The Witcher computer game
During his visit in the Outskirts of Vizima, the witch Abigail tells Geralt about the time Alzur the mage met a beast immune to magic. The beast asked the mage a question, and by answering correctly Alzur was able to defeat it. Thenceforth, the beast was known as "Alzur's demon".

Berengar's notes on the Beast confirm that the Beast haunting the Outskirts could be "Alzur's demon".

Over the course of the game, Geralt also comes upon a book called The Double Cross of Alzur, which reveals how to create a koshchey.