Once & Future (Once & Future, Bk. 1)

When Ari crash-lands on Old Earth and pulls a magic sword from its ancient resting place, she is revealed to be the newest reincarnation of King Arthur. Then she meets Merlin, who has aged backward over the centuries into a teenager, and together they must break the curse that keeps Arthur coming back. Their quest? Defeat the cruel, oppressive government and bring peace and equality to all humankind. No pressure.