Dragon Sword by Mac Flynn

Rose and William Thorn have finished one adventure and begin another, this time in the land of the fabled elves.

Unfortunately, their arrival in the vast woods of the fair folk is greeted with less hospitality than they had hoped. A blight has fallen on the green woods of the fair elves and suspicion has fallen on all outsiders. Only Will’s old association with them keeps the pair from being thrown into the grassy pit of a dungeon.
To make matters worse, they’re not the only new arrivals. A pair of humans stumbled into the woods searching for something they won’t name. Their suspicious behavior combined with their unpolished nature landed them in the dungeons. Their warnings of danger fall on the preoccupied pointy ears of the elves who are desperate for a solution to their problem.
That solution comes in the form of a famous elf lord who arrives eager to study the blight. The elves are overjoyed but Rose finds herself mysteriously repulsed by the fair-haired newcomer.

As the troubles mount along with the tension, Rose and Will must find a way to navigate politics and suspicion to find out the truth before their shadowed foes deal with them.