In the wilds of Skellige you may encounter a mysteriously empty worksite.
The Witcher 3: Abandoned Sawmill
This quest is a little bit hard to find; it's so far from anything of note that it has its own fast travel signpost. Its located west and slightly south of Gedyneith, so set out into the forest and look for trails leading to the site.
Optionally search the southwest stone hut, breaking in the door to find a letter; the main building to the north, to find some notes; and the shed to the northeast, to find some letters. Read them all and draw conclusions about a nearby threat.
Whether you do all that or not, look for some human bones near a barn. Examining these immediately triggers the appearance of a Leshen, so quickly apply Relict Oil if you haven't already, and go after it.
Killing the Leshen resolves the quest. There is no reward, but do loot its corpse.