Echo's Answer

Lachlan, a Changeling in the service of the Lady of the Lake, meets the nymph Echo when he travels to Dodona Forest on a mission of peace. When things go wrong, Echo is cursed, and Lachlan goes mad. Each is the salvation of the other, but war, time, and distance only drive them apart.

This is the nice/sweet version, with a happily-ever-after story with no cheating.

Echo's Answer: Lachlan is part of the Strygoi Witches & Vampires universe. It can be read as standalone, but if you're familiar with the series, you'll recognize characters and events from Viktoria's Shadow: Jael, Bijou's Cure: Zeke, and Viktoria's prequel, Shadowy.