Mastering the Flames by SJ Himes

Guilt-ridden after the massacre of his family, Isaac Salvatore turned to binge drinking to escape the pain. Now twenty-four years old, Isaac is a recovering alcoholic woefully out of practice in the magical arts, leaving his fire affinity hanging on the edge of disaster. After a month of rehab, he returns to Beacon Hill and his family, determined to remain sober, learn to control his magic, and figure out a plan for his life that doesn’t involve drinking.
Constantine Batiste is the oldest, most powerful vampire in the city. Born in ancient Gaul, the bastard son of a Celtic king, his long life has been shrouded in tragedy and horrors. Recent mistakes have left him wary and determined to guard his clan from all foes. When two of his clan members fall victim to an ancient evil, he summons the Necromancer of Boston for aid. Accompanying his older brother to the Tower is the handsome young fire mage once wounded by Constantine’s arrogance, and their encounter reignites an attraction that burns within both Constantine and Isaac.
The answer to who is targeting the vampires of Boston is buried in the dark, early days of Constantine’s transition to an immortal life. Isaac finds himself saddled with a painful insight into the evil cutting a swath through the supernatural population of Boston. While his brother, Angel, takes over the hunt to find and stop the threat to the city, Isaac struggles to find a balance between helping his brother and finding his own purpose and place in the world, free from his brother’s shadow.
Falling in love wasn’t part of his plan, but mastering the flames that burn between him and Constantine soon becomes the most important thing in his life, even as an ancient evil seeks to destroy them.