Black Is Magic by JC Andrijeski

Black’s old army buddy, Cal, needs Black’s help on a new case––a bizarre murder in San Jose, California, where a man is found sawed in half. The victim, a wealthy venture-capitalist and soon to be brother-in-law of Black’s friend, disappeared right before his fortieth birthday party.
The murderer may also be something not-quite-human.
Holed up in Santa Fe with his wife, Miri, waiting for another friend to recover from a near-fatal injury, Black has time to kill, and he owes Cal.
But nothing about this murderer or their victims is what it seems.
When their main suspect takes out Black and Miri single-handedly, destroying a building in downtown San Jose, it sets off a series of events that could have significance for the entire world. Worse, an old enemy stands ready in the wings to take advantage of the situation, and get his revenge on Miri and Black.
Killing them might be his final act before he remakes the world in his own, twisted image. Only this time, he’ll have the creation’s two most powerful beings at his side.