Inferno of Darkness by J.L. Weil

Lexi Winters thought she’d experienced loss and heartache before, but nothing prepares her for the raw pain at being separated from Ashor Clave—the demon she is soulbound with. Nor does she expect the strange darkness that awakens inside her.

The Queen of Darkness has not given up on her plan to conquer the underworld, an epically bad idea on so many levels. If the queen succeeds, a rift will be created in the barrier between worlds, allowing demons to walk freely. Her first attempt might have been thwarted by Lexi’s escape, but the queen is ancient and patient. She has a bargaining chip—Ashor.

Lexi wants only to save her mate, which involves breaking him out of the underworld, but there are so many complications she never sees coming. Lexi never thought she would ever go back to Hell, but for Ashor, she is willing to risk more than just her life. Because when you are dealing with the queens of Hell, your soul is on the line.