This page of our Ghost of Tsushima guide contains a detailed walkthrough for A Gathering Storm story quest. It is a somewhat short mission, although completing it requires stealth. From our walkthrough, you will learn how to sneak into the lighthouse without being noticed, and how to successfully scout the Izumi Harbor.
Sneak into the lighthouse
This mission will be unlocked once you complete Wolves at the Gates story quest. Once again, you must return to Jogaku Temple and meet Yuna. This time, the woman will be standing next to one of the temple buildings. After the conversation, the action takes you to the nearby Izumi port, where the main Mongol force is staying.
You must avoid activating alarms from the very beginning of the mission. Start heading towards the Jogaku Lighthouse, which is visible in the distance. Stay crouched and use focused hearing often. You'll run into a few guards along the way. They can be easily killed with stealth attacks.
Opponents from the encampment can also be attacked from behind or you can simply sneak past them - the latter option requires you to walk along a hanging line leading to the roof of the tent.
After getting close enough to the lighthouse, wait until two guards stop at the main entrance. Take advantage of the fact that they're not patrolling the area and sneak into the back of the lighthouse. Squeeze through the hole shown in the screenshot to get inside the lighthouse.
Investigate the barrels
Stay on the ground floor of the lighthouse and interact with the barrels - Jin will determine that there is poison inside of them.
Scout Fort Izumi
Use a ladder to get to the top of the lighthouse. Fortunately, there are no enemies there.
Begin scouting - take a look at the rocks on the left, the empty part of the village, the hill that can serve as a good spot for archers, the birds flying over the harbor, and the harbor master's estate. Listen to the conversation with Yuna to end this quest.
Rewards for completing the mission: Legend increased (xp), Heart of the Jito story quest unlocked.