|Finish Cyrus' Chapter 1||Palace in Atlasdam||13,000 Leaves|
You will find the questgiver for this inside of the palace in Atlasdam, which is the area north of the one with the library. Depending on when you first get this Side Story, you might want to wait to attempt to finish it. Before even thinking of completing it, though, you will need either Tressa or Therion (Tressa is recommended). Waiting until you're towards the end of Chapter 1 or the beginning the Chapter 2 is ideal, since you will need to traverse a Danger Level 20 area to get to the necessary NPC.
The specific NPC you are looking for is found in East Saintsbridge Traverse, which is close to Clearbrook. Fast travel to the town and exit to West Clearbrook Traverse, then start heading south to get to South Clearbrook Traverse. Here, you want to go east to a fork, then south to another fork, where you will be going west to eventually get to East Saintsbridge Traverse. In this area, head south to a crossroad, then south to find the target NPC on a bridge. You can either Steal or Purchase the Final Masterpiece from him, but it's probably just easier to Purchase it. Bring the painting back to the NPC in Atlasdam to finish the Side Story.