Northern Ruins (600 A.D.)

Enter the Northern Ruins in 600 A.D. Go west and down the stairs.

In the next room, go south and defeat the 2 Ghost Knight enemies.

Go east and defeat the Sentries.

Go north and open the chest for a Turbo Ether. (Note: Chests in 600 A.D. can be re-opened in 1000 A.D. for the same items, and then again in 600 A.D. for the same items.)

Return to the previous room and go west and north.

Head east through the hall and up the stairs.

Enter the room at the top and defeat the Fallens and Flesh Reapers.

Return to the previous room and exit the ruins.

Go south to Choras and enter the southmost Residence. Pay 2000G to repair the Northern Ruins.

Return to the ruins. Go west and down the stairs.

Go around the room and head north through the door into the next room.

Go north through another door to reach Cyrus' grave.

Put Frog as the leader of your group and check the grave. Cyrus' ghost will appear and speak to Frog, upgrading the Masamune, which will be automatically equipped on Frog.

Exit the room and leave the ruins.

If you revisit Cyrus' grave in 1000 A.D. after upgrading the Masamune, you can find a Strength Capsule in the northwest area of that room.

Head south to the Residence and pay 2000G again to repair the ruins.

Return to the ruins and go east, then upstairs.

Enter the left room and take the Elixir from the chest.

Enter the right room and check the sealed box, but do not take its contents yet.

Go north into the middle room, and continue north through the next couple of rooms, defeating enemies.

At the northmost point, check the two sealed chests, but do not open them yet.

Go back through the previous rooms and exit the ruins.

Warp to 1000 A.D. and enter the ruins (now called the Hero's Grave).

Go back to the rooms with the sealed chests and claim the upgraded items: a Valkyrie Bow, a Suzaku, and a Moonbeam Armor.

Leave the Hero's Grave and return to 600 A.D.

Re-enter the ruins and go to the same rooms.

Defeat the enemies and open the sealed chests to get an Onimaru, a Siren's Kiss, and a Nova Armor.

Exit the ruins; the side quest is complete.