Phoenix Night by LC Taylor

After Phoenix Night’s partner died in the line of duty, something changed inside him. He left the force, using his talents to catch cheating spouses in the act as a private investigator. Being a PI is easy work. Letting him work through the weird changes he’s been going through since his partner’s death.
Changes no one can explain…
An old colleague from the force asks for his help solving a string of murders involving young women. Every one of the victims has one thing in common – a bizarre symbol left on their bodies. Agreeing to help Phoenix begins to uncover something dark and sinister. He realizes there is more to the world than what is seen, making him realize what he’s becoming.
And if that wasn’t enough, he agrees to help Aurora Black find her missing sister. The more he digs into Aurora’s sister’s mysterious disappearance, the more he realizes it may be connected to the murders – and possibly what happened to him the night his partner died…
The night everything changed.
Aurora is slowly breaking down his walls. He swore off letting anyone close, for fear of losing someone again. But the more she pushes, the more his walls crumble.
But what happens when Phoenix discovers what Aurora is hiding?
Phoenix knows this… he won’t stop until he has the answers.
All of them.