The Book of Legends by Carmen Rosales

Two Brothers divided by blood
Elarion is a world ruled by dragon gods. Kingdoms are at war, rivers run red, and legends of lost souls linger like a chill in the air.
Cursed by the dragon gods for their failure to keep peace, the two kingdoms suffer. Elariya, ruled by the merciless King Therion, endures an eternal winter. Across the River of Blood lies Nythia, shrouded in darkness and ash, where his exiled brother, the dark Prince Kainen, waits for revenge.
When a strange human appears on his lands, Kainen suspects she’s a sorceress sent by Therion to destroy him. Yet her fierce defiance and sultry curves threaten him in ways magic never could.

She was the key, the flame between them.
College sophomore Selene has always felt out of place. After the death of her aunt, heartbreak leads her to do something she usually avoids: looking in a mirror.
But the antique that belonged to her aunt is no ordinary mirror. It pulses with supernatural energy and pulls her into a world beyond her wildest dreams.
She ends up in Nithya, where Kainen, the warlock and dark prince, imprisons her. Desperate to return home, Selene can’t help how Kainen’s power unravels her or her undeniable attraction for him. She begins questioning who she is, sensing Kainen may hold answers she never thought to seek.
In a world ruled by mythical creatures, where loyalty is rare, and trust could prove deadly, desire may be her greatest danger of all.