The Crown of Dalemark - Diana Wynne Jones

When this final book of Diana Wynne Jones's quartet of novels about the mythical kingdom of Dalemark was published in 1995, it earned the Mythopoeic Fantasy Award for Children's Literature.
The Crown of Dalemark continues the adventures of Mitt after his flight from Holand as a fugitive accused of attempted murder. Since his arrival in the North of Dalemark, Mitt has become disillusioned. The North seems no more free than the South from which he came. And now he has been given an order to kill someone he doesn't even know, or else risk the lives of his friends.
Forced once more to flee, Mitt is joined by Moril, the quietly powerful musician, and Maewen, out of her time but mysteriously fated to play a part in their quest. For the evil powers of the mage Kankredin are re-assembling, and only the Adon's gifts-the ring, sword, and cup-can reunite Dalemark.
The Countess and Lord Keril send Mitt to kill a young woman Noreth Onesdaughter, who claims to know where the lost crown is hidden.