The Coupling by M. J. Fitzmaurice

An ancient demon, thriving on human death is unleashed from an artifact found in South Dakota. An archeological technician, Rhonda Steppe, bonds with it and learns how intoxicating it can be to kill. The demon, Night Warrior, can only appear in the darkness. Together they begin a bloody trip west to a home that Night Warrior is called to.

Shasha Harrings and her specialized team try to run Rhonda to ground to end their killing spree. They follow Rhonda and demon into a complex series of caves. The demon is going home, and Shasha can only speculate on its intentions.

Shasha is mixed race with jet black hair and pale skin, she has her mother's dark oriental eyes. She comes from money is the leader in any group situation.

David Seskin is a Special Agent with the FBI. He is physically imposing and keeps his dark hair in a square crewcut. He's often compare to Wesley Snipes. When he's not chasing bad guys, he likes to dress in a clown suit and entertain children in the hospital. David has an ability few people know about. He can read a person's emotions.

Graciella (Grace) Ortiz is undercover with the DEA when her lover, Barbara, is killed by Rhonda and the demon. She's tall for a Puerto Rican. She, despite her sexuality, is devoted to the Blessed Mother Mary. While she talks like a street kid, she's well educated and despite her small stature, is ferocious when something needs doing.

Merle Huegenot is a trans woman who talked her CIA supervisor into paying for her transition surgery when she discovered the woman he was having an affair with was really an Egyptian spy. She is tall, blond, with dark eyes and a kind disposition. Her contacts and analytic skills round out the team charged with discovering and stopping harmful paranormal activities.

The Coupling is book 1 of the Shasha's Team series.