I read this circa 1997-98 and it was from a used bookstore even then. Unfortunately, I lost track of it around 2003 and I don’t recall the title or author but I remember the plot pretty well.
It’s fantasy novel about a wizard; book starts with his training difficulties and at some point he gets terrible burn scars on his back. Time jump to him on a quest with other companions in search of a chalice. A noblewoman running from home stumbles into the mess but her hidden gift allows her save his life when he’s overwhelmed by the chalice’s defenses, though they are now bound together. After they both recover (separately; she was returned to her family) in the capital city, the country is at war from an attacking neighbor. The noblewoman makes friends with a young girl with a large purple birthmark on her face; she has also developed more magic, though she had always been able to talk to trees (she had run away from home because of her abusive father who hated magic). When the royal family is killed by a poisoned mist, the noblewoman’s young friend turns out to be a princess and becomes the next ruler. The noblewoman finds herself elevated to royal advisor.
The previous king didn’t trust the wizard because he was afraid of him, but the princess-turned-queen will listen to him because she trusts the noblewoman. The noblewoman moves into the wizard’s tower to learn magic; meanwhile the war is still going on.
In the end the wizard uses the chalice and sacrifices himself to defeat the enemy’s powerful mage. But due to the bond between them, the noblewoman-turned-wizard can still see him in the “other world” so she’s not alone.
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