The Witcher: sword of destiny. Andrzej Sapkowski. 2016.

Book 6 in the series.
Gerald is a Witcher, a man whose magic powers, enhanced by long training and a mysterious elixir, have made him a brilliant fighter and a merciless assassin.

His targets are the deadly monsters which ravage the land and attack the innocent. He roams the country seeking work, but comes to realise that while some of his quarry are unremittingly vile, others are the victims of sin, evil or simple naivety.