From the back cover:
"There's only one way to get there - through the Barrier Peaks. Once you're there, you are confronted with armed guards, traps both magical and mechanical, and legendary beasts...not to mention the wandering necromancers.
Why go in there? Because rules of neighboring countries have asked you to persuade the Black One to help rout that group of expatriate necromancers. They tried a coup, failed, and came to the Vale of the Mage to hide - and, presumably, to regroup and ally with the Black One. They may have gained the help of the Tree People, as well; you won't know until (perhaps) it'll be too late to turn back. It's a jungle in there..."
From the Introduction:
"It is intended for characters of 7th through 9th level, for a total of 35 to 40 levels. A mix of character classes is desirable. Further, it is important that at least some of the characters have magical weapons."