Mage Errant by John Bierce has a main warlock character who pacts with a greater power. There are other ways of pacts as well that feature throughout the series. This is one of my favorite all time series, particularly for the main group of teenagers who support each other no matter what. The magic system is great and worldbuilding starts to shine as the series progresses.

Some recommendations for novels based on Greek mythology (though I haven't read them yet):

• Soldier of the Mist series by Gene Wolfe
• Troy trilogy by David Gemmell