Shadows of the Eternal Flame, Written by Nova Weaver.

Elena Harper, a quiet London museum curator, is thrust into the magical realm of Aetheria after discovering an enchanted locket. There, she meets Marcus, the immortal guardian of the fading Eternal Flame, the source of all magic in his world. A prophecy reveals Elena is the key to restoring the Flame, which is dying under a curse from the banished Shadow King. As she awakens her own dormant powers, a forbidden and dangerous romance blossoms between her and Marcus. Their quest is challenged by Victor, a rogue guardian who wants to seize the Flame's power for himself. Elena learns her own lineage connects her to shadow, making her union with Marcus potentially catastrophic. In a final battle, she must embrace her dual nature to defeat Victor and confront the prophecy. She saves Aetheria by merging her essence with Marcus's, stabilizing the realm but sacrificing her normal life. Now a permanent "bridge-walker" between worlds, Elena accepts her new destiny as a new threat emerges on the horizon.