By the Light of the Heart Achievement in Octopath Traveler II

Crossed Paths are new optional story segments that appear on the map once you have completed a pair of character Chapter 2 story quests. Simply travel to the appropriate marked town to begin the quest and complete it, or start it from the Tavern later. The Neoseeker guide has links to all of the crossed path quests if you need help to complete them for any reason.

Note: While the Crossed Paths are optional to complete the game's story, all four have individual achievements and are all required to unlock the game's superboss for Gate to the Netherworld (100G).