Lawrence Watt-Evans 
The Spriggan Mirror [EPUB ebook] 

Every wizard in Ethshar knew that if you needed something special, something difficult to find, that Gresh the Supplier was the man to see. He was expensive, but he always delivered. So when the Wizards’ Guild finally got fed up with the little green nuisances that called themselves 'spriggans, ’ the Guild hired Gresh to fetch them the magic mirror that created the troublesome imps.

The wizards thought finding it looked impossible. Gresh thought his methods would do the job.

But no one had asked the spriggans what they thought!