Shield of Fire by Keri Arthur

A new relic hunt. A new evil...
Resident Relic Hunter for Deva's Fae Council is a title Bethany Aodhán never wanted, but it isn't like she has a choice. And in truth, the appointment might just aid her quest to find her mother's murderers—because at least one of those responsible is a member of the council, and getting access to their records could give her the connection and their identity.
But before she can be given her first task or even start her behind-the-scenes hunt, a building belonging to Cynwrig Lùtair—the highborn dark elf who is not only her council liaison but lover—is set alight, and they barely escape with their lives.
It quickly becomes clear that the cause of the fire was a relic belonging to the god of smiths and fire, capable of destroying anything made within a forge, old or new. And the man wielding it is an elf who stirs uneasy echoes of past darkness within Bethany.
As the attacks intensify and draw ever closer to home, it's increasingly evident that not only do the keys to finding the culprits lie in the past Bethany can't remember, but he's someone who knew her very well indeed.
Someone who has no qualms about doing whatever it takes to get what he wants from her.
Even if that means killing Lugh.