The Magician's Apprentice (The Black Magician Trilogy 0.5) by Trudi Canavan

Taking place hundreds of years before the events of The Magicians'

Guild, The Magician's Apprentice is the new novel set in the world of
Trudi Canavan's Black Magician Trilogy.

In the remote village of Mandryn, Tessia serves as assistant to her father, the village Healer. Her mother would rather she found a husband. But her life is about to take a very unexpected turn.
When the advances of a visiting  Sachakan mage get violent, Tessia unconsciously taps unknown reserves of magic to defend herself. Lord Dakon, the local magician, takes Tessia under his wing as an apprentice.

The hours are long and the work arduous, but soon and exciting new world opens up to her. There are fine clothes and servants - and, to Tessia's delight - regular trips to the great city of Imardin.

However, Tessia is about to discover that her magical gifts bring with them a great deal of responsibility. For a storm is approaching that threatens to tear her world apart.