The Hero's Grave (also known as Northern Ruins before the quest of Cyrus in the SNES/PS version) is a location in Chrono Trigger. Situated on the island of Choras, it appears as a large castle in the Middle Ages and the Present. The red roof from the Middle Ages changes to blue during the Present.
After reacquiring the Epoch from Dalton, Crono can fly to the island of Choras, East of Truce. There, surrounded by the forest stands a ruined castle that houses the grave of a long-dead knight, Cyrus. When the player first arrives, they find the way barred by massive holes in the floor and a sword-wielding apparition. Untouched by spell or blade, the battle turns into a fight to stay alive. After a few turns, Frog realizes the ghost belongs to his long-dead friend -- the knight who is buried in the ruins beyond. The ghost dissolves and the party is booted from the scene.
To fully explore the ruins and power up the Masamune, the party must hire the Carpenter in the year 600 A.D. In order to hire him, the Carpenter's Tools must be acquired from another carpenter from the year 1000 A.D. who lives in the same area. Once the tools have been given to the carpenter in 600 A.D., send him and his crew to the ruins to do repairs. Follow them there and clear out the monsters. Hire the Carpenter again for a small fee and Cyrus's grave will be accessible. After powering up the Masamune leave and hire the carpenters again to clear out the upper level.
NOTE: You can open the sealed chests here, but it's not advised. Simply inspect the chests in the upper level and decline to open them. After inspecting all the chests return to the Hero's Grave in 1000 A.D. and open the sealed chests then, as the items inside will have been "powered up". One of them will be the legendary Moonbeam Armor.