Beneath the Windowless Tower (2013)

From the first page:

Malazar the Mage, like all great wizards, was a noted eccentric. His particular obsession was the collection of certain rare and bizarre antiquities, unearthed at great time and personal expense from long-forgotten buried cities where saner men feared to tread.
It was on his final such expedition that Malazar reputedly uncovered a jewel of great value and magical power, the so-called Egg of Desire. In an area of remote hills, he constructed a high tower in which to house his prize – a tower with neither windows nor doors.
As the years drew on, Malazar began to disdain city life. He withdrew more and more to his strange, windowless tower, conducting his business by emissary. It is now more than a decade since there has been any word from either Malazar or his agents, and persistent rumour among the more knowledgeable circles of thieves and treasure- hunters has it that the mage is dead, and within the tower his jewel lies free for the taking - if only there were some way to get inside...