The Unseelie Court by Kathryn Ann Kingsley

Some webs are meant to be broken.
Ava Cole thought she’d escaped the Web—and the dangerously seductive spider who ruled it. She was wrong.

Now the “guest” of the Moonlit Court of the Unseelie, Ava finds herself caught between two immortal brothers preparing for an ancient war. Valroy, the vicious Unseelie King, offers her power in exchange for becoming his weapon. Serrik, the exiled spider-like fae whose dark magic flows through her veins and haunts her dreams with promises of love and vengeance.
But as the Web’s influence spreads through Ava’s body, marking her with strange and twisting tattoos, she discovers that everyone has been lying to her. The truth about what she’s becoming is darker than she ever imagined.
When Ava learns the devastating secret at the heart of the Web, she’ll face an impossible choice: risk her humanity to become an unstoppable force of destruction…or fight to remain herself and risk losing everything.
Including the fae exile who might have stolen her heart.