Alzur was a famous mage and the inventor of many spells, including Alzur's Double Cross, Alzur's Shield and Alzur's Thunder. He is also known for having created many horrible monsters, some of which still roam the world. He is said to have had a voice and diction which were positively mellifluous. His spells were also used by other mages. Notably, Fregenal may possibly have used the spell known as Alzur's Double Cross to create Koshchey.
A very old and influential mage named Ortolan mentioned that Alzur, along with his master and teacher Cosimo Malaspina created the mutations of the witchers. It is alluded that Alzur did the lion's share of the work though.
During his visit in the Outskirts of Vizima, the witch Abigail told 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".
Over the course of the game, Geralt also comes upon a book called The Double Cross of Alzur, which is an edited and censored edition of Alzur's original work which revealed how to create a koshchey.