Save Theresa from the Cumans
*** Spoiler - click to reveal *** While escaping Skalitz and trying to find a horse, you'll hear Theresa yelling while being attacked by Cumans. Before you get on a horse, whistle at them with X (Keyboard) or just run up and punch one of them to distract them from Theresa. They are incredibly powerful at this point, but luckily you don't have to beat them, the achievement triggers immediately and from here you can either die or escape on the horse. Theresa will later thank you for saving her life.