Looking for a teen fantasy novel from when I was a kid, unfortunately I can't remember the title or the author. I'm sure it was part of a young adult fantasy series though. Not a trilogy or anything, just a part of a series of individual publications like Point Fantasy. It told the story of 3 adventurers: a warrior, a wizard and a small rogue. The antagonist is a shape-shifting monster. The monster is searching for a magical gem of incredible power and the party of adventurers are trying to stop it.
I seem to recall the cover depicting a landscape, perhaps with a character or two in the foreground looking over it (?). I can remember the cover had a red border.
It was approx 300 pages.
I can recall some scenes:
I think the journey starts with the warrior, alone. He meets the wizard and rogue along the way.
In one section the party is searching a series of caves and are forced to escape after hoards of orcs/goblins discover them and give chase.
There is a beautiful female character that the party discover in a forest clearing. The rogue character falls hopelessly in love with her. Her name is Star. Her origin remains a mystery until the climax when it's revealed she is a shape-shifting monster as well.
One scene takes place in a hot-air balloon. The heroes are trying to escape and this is the moment the beautiful female character reveals her true nature and leaps from the balloon.
The wizard character spends the majority of the story struggling to channel his magic. He often tries to invoke spells that fizzle out. It's not until the climactic battle that he rediscovers his magical prowess.
The final battle between the warrior and wizard against the shapeshifter takes place in a forest glade. The monster has the magical gem and has become immortal, constantly healing its wounds. When the wizard gets his hands on the gem, he is able to tap into its power. He sends the monster flying with just a glance, then he empowers the warriors sword with a white-hot light that prevents the monster from healing his wounds. With the last power within the gem, the wizard captures the monster in a fast-growing tree, sealing it within.
The novel has about a dozen black and white illustrations. I remember the monster depicted as thin, wiry being with many horns and spikes over its body and head.