The Traitor's Tale (World of the Frostborn short story)

A thousand years ago, the sorceress Antenora betrayed Arthur Pendragon and the last Keeper of Avalon, but was cursed by dark magic for her folly. 

After a thousand years of regret and pain, Antenora is desperate to redeem herself...and her chance has come at last.

But the Frostborn stand in her way, and even all of Antenora's magic might not be enough to overcome their terrible power...