To the bottom left of your fortress, there is a unique mark shaped like a house — Swamp Witch’s Hut. Head there, at the upper right corner you’ll meet two fairies — Falchos and Tiressia.

Speak to them, they will ask you to kill the Scythe-Tree, who destroyed the village. The monster is in the lower right corner of the place.

After killing it, go back and report on finishing your task. You receive 450 experience points as a reward.

https://www.youtube.com/watch?v=elvUEHmXcFM