We've offended him, Sven. The Woodland Spirit seeks revenge. This here is him saying we've strayed from the old paths. –Harald of Fayrlund
Some monsters are so powerful local populaces begin worshiping them as gods. Such was the case with the leshen dwelling in the woods near the Skellige village of Fayrlund. The elders were absolutely convinced the monster watched over them by defending them from their enemies and training their hunters.
The witcher had his doubts, however. Leshens are mean, self-serving creatures - and the ancient one living in the woods near Fayrlund was surely no exception.
Geralt knew freeing the villagers from their supposed protector's grasp would be no easy task - if he decided to even try. The leshen known as the Woodland Spirit was so powerful as to have mastered the art of marking. This meant the monster had inserted his essence into one of the residents of Fayrlund. As long as this unfortunate soul remained alive and near the settlement, the leshen would always be reborn near its lair.
Yet even with the marked one eliminated, killing the leshen would be no easy task. The monster commanded the obedience of the denizens of the woods - it could thus call on wolves, ravens or even the trees themselves for help. Luckily, it had a weakness as well: it was vulnerable to fire, that born of the Igni Sign in particular.
If Geralt lets it live:
In the end, however, there never was a battle between Geralt and the Woodland Spirit. Geralt knew that the leshen was surely less benevolent than the elders thought - but he also knew killing it would do more harm than good.
If Geralt kills it:
The Witcher managed to kill the leshen, ending the murders terrorizing the woods around Fayrlund. Yet this did not mean the villagers' troubles were over...