Forever Dusk (Mortal Machine, #3)

Since their last battle, life for the Mortal Machine—the secret-society that protects Earth and its inhabitants from dark outside forces—has become almost...normal. For everyone but Addison. The evil she’s imprisoned in her soul has begun eating away at her sanity, and despite her soul mate Asher’s efforts to hold her together, it’s causing painful and terrifying delusions.
Consequently, nobody believes Addison's warnings that Marcus, their old enemy, has returned. When Marcus threatens Asher and the Machine, she agrees to find what he seeks—a treasure that, in his hands, could be deadly.
If she relies too much on Asher, she knows he’ll likely imprison her, if only to preserve what's left of her deteriorating mind. But if she fails this final test, Earth will fall. So Addison is forced to distance herself from her love, to prevent the life she wants with Asher from being over before it truly begins.