The Rise of a Dark Mage — D.L. Harrison

Cassandra is a dark mage in the kingdom of Zual, she's also a mage prodigy.

She hates both her kingdom, and her master. She wants him dead, not to take his place, but so she can escape the bond of magical slavery, and leave Zual behind to explore the other kingdoms of the world. Her ambition will drive her to rediscover the secrets of the strongest of magic.

She is determined to succeed, or she'll die trying.

Note: This stand-alone fantasy book follows the life of Cassandra, it takes place in the same world as the stand-alone book The Formerly Dark Mage, but happens three hundred years later, long after Silvia is gone, and some shocking changes have taken place in the world.
It is not necessary to read the first book, but if you plan to read both, you should read The Formerly Dark Mage first to avoid spoilers.