Calm Before the Storm is one of the shorter missions in Assassin's Creed Odyssey: Legacy of the First Blade. It is based only on having conversations with people.
The first thing you need to do in this mission is to meet people in a boat. After talking to them you will get the next part of the task.
You have to go with Darius' descendant to their old home. There you will have a long conversation.
Your last task in this mission is to talk to Klet. A descendant of Darius, Barnabas, and Darius himself, are also included in the conversation.