The Hundred Doors. The Immortal Tyrant, #1

The world of Arundel had barely recovered from the Great Cataclysm when Thoryll rose to power as the Empress. She began to rule with an iron hand and began to control the other realms. She grew old and was dying before her work was complete. She discovered a method of achieving immortality mere hours before her passing. It also restored her youth and allowed her to bestow immortality on those closest to her. She ruled another thousand years, growing more forceful and tyrannical until a Scholar training as a Mage discovered a method of both removing Thoryll's immortality and from power.
With a group of willing and like-minded adventurers, she set out to achieve that goal, but first had to locate and learn the secrets of the Hundred Doors.