The Heir of Night (The Wall of Night Book One) by Helen Lowe is currently 99c in the US
This was previously discounted some time ago and I bought it then and enjoyed it and bought and read books 2 and 3 in the series and am waiting for the last in quartet to come out.
https://www.kobo.com/us/en/ebook/the-heir-of-night-1
https://www.amazon.com/Heir-Night-Wa.../dp/B003V1WSL4
“A richly told tale of strange magic, dark treachery, and conflicting loyalties, set in a well realized world.”
—Robin Hobb, author of Dragon Keeper
A DARK ENEMY. AN ANCIENT WAR. A NEW CHAMPION.
Young Malian is being trained to rule. Her people garrison the mountain range known as the Wall of Night against an ancient enemy, keeping a tide of shadow from the rest of their world. Malian is expected to uphold this tradition, yet she's known little of real danger until the enemy attacks her fortress home and the Keep of Winds becomes a bloodbath.
Malian flees deep into the Old Keep - and when the danger is greatest her own hidden magic flares into life. But if she accepts its power, she must prepare to pay the price.