I remember reading it in the 90s, so it was written then or earlier. The book's story is set in a medieval world.
The main character is a human boy who had his arm replaced with a enchanted one, and his companions are his vampire girlfriend and a barbarian woman whose skin is slowly turning to stone from a curse put on her.
The group are fleeing an evil brother/sister sorcerer couple who are trying to stop the boy from fulfilling some prophecy tied to his magic arm.
The cover of the book had the boy touching something (I think a staff?) burning his hands, his vampire girlfriend covering her face from the glow, and the barbarian woman standing behind
The book opened with the trio on a pirate ship; the crew tried to betray them but were killed. They eventually moved onto some lost underground city. The boy tried to read some old books but because of age they turned to dust, and lastly I read they got caught on a beach where the barbarian's stone skin made her sink in the sand and eels attacked her until the vampire ran into the water naked and the eels died when they touched her undead skin
Meanwhile the twin villains were using captured human women to breed with lizardmen and make hybrids, and something about airships using waste methane to fly so they always stank of that.