From PlayStation Blog:
"Legends is an entirely new experience — it’s a separate mode that doesn’t follow Jin or the companions from his journey, but instead focuses on four warriors who have been built up as legends in stories told by the people of Tsushima. Ghost of Tsushima’s single-player campaign focuses on an open world and exploring the natural beauty of the island, but Legends is haunting and fantastical, with locations and enemies inspired by Japanese folk tales and mythology and an emphasis on cooperative combat and action."
Pretty awesome news! Can't wait to try it out.