Robert Lynn Asprin

As Court Magician to Rodrick the Fifth of Possiltum, Skeeve must oppose the universe's mightiest invading army, but he is not sure how, and his mentor, Aahz, who lost his sorcerer's power, cannot help.

Skeeve was a magician's apprentice—until an assassin struck and his master was killed. Now, with a purple-tongued demon named Aahz as a companion, he's on a quest to get even.