To access the Ghost of Tsushima DLC, you must first complete Tales of Iki, which are the new quests available. But it's not accessible until you get to act 2 when you access the Toyotama area.
The tale/quest you need to complete is called Journey to the Past, where you need to investigate a settlement at the Drowned Man's Shore, northwest of Akasmima Village in Toyotama.
The fastest way to get there is to quickly travel to the nearby Fox Den southeast of Drowned Man's Shore, then head northwest to get to where to start the quest. Once you arrive at the settlement, you need to explore the area and examine the clues to progress in the quest.
A trigger will eventually occur when the Mongols start attacking with a new type of enemy, a Shaman. This new enemy sends the Mongols into a trance of bloodlust. Send the shaman as quickly as possible to prevent him from buffing the Mongols advancing towards you. Once killed, more will spawn. After the battle, investigate the Mongol Shaman, and they will mention Iki Island and the Eagle. A cutscene will play and Yuna will join you.
Speak to the boatman nearby and they'll take you to Iki Island, where you can explore Ghost of Tsushima's new story-based DLC.
For more Ghost of Tsushima Iki Island guides, we've got you covered.
