Tristin is a duchy that is at war with Askaria, therefore it can be said to be quite powerful as a nation in the S&S universe. It is located on the northern coast of the western continent.
Its main claim to fame is that it was once ruled by Duke Admont, a.k.a. Carsejaw the Cruel, who calmed the unrest of the land by blaming innocent "witches" and sending them to be executed. He also created the Ducal Guard, a fearsome group of knights who enforced his rule.
The Pitchwoods, an area in the game, resembles a location in Tristin, and Carsejaw can also be fought there.
Sir Francis the Resolute was also from Tristin, who later became the Sodden Knight upon undeath.
Based on the Pitchwoods and the appearance of the denizens of Tristin as very pale and creepy, it seems to be a quite forboding place.