Help me find the name of the quest (The Witcher 3: Wild Hunt)

On Skellige I found the corpse of a man and a horse, after examining them, someone threw a stone from a cliff. In search of the culprit, I found a cave in which there was a lot of endriag and a poisonous head-eye. Further in the cave lay a shoe in the shape of a naker's foot and also drawings with a plan to kill Geralt.

No marks were made in the journal, tell me where to get the quest or how to finish it