Tender Blackguard by Amy Sandas

A Servant with Secrets.
A childhood in the rookery taught Lark Evans two things; how to survive a perilous world and people deserving of loyalty are precious few. When her friend goes missing, Lark will risk it all to find her. But the shadows of Mayfair are dangerous and her handsome new employer might just be the villain she’s seeking.

A Lord Born of Darkness.
Alastair Blackwell never wanted to be the next Marquess of Warfield, but upon his father’s death, he discovers a secret that changes everything. To purge the demons of his past, he must destroy the arcane society his father founded long ago. His greatest challenge could prove to be his impertinent new housekeeper.

A Sinister Evil Thriving in Mayfair.
Though Alastair vows to keep Lark far away from the evil she seeks, he can’t help but wonder if she could be the key to ending it all.