Side: Templar (Central Command)

Begin the sidequest by speaking with Grand Marshal Braev Lee in the Sanctum in Gathelatio. He tasks Edea with retrieving either the Grand Marshal's Sword or the Grand Marshal's Shield.

To obtain the Grand Marshal's Shield, travel to Eternian Central Command. You can reach Eternia City via teleporking and then travel west out of the city, following the mountain trail north.

Once inside Eternian Central Command, proceed north-west through the newly opened door to reach the prison.

Prison B1: From the entrance stairs, head south to find a chest with an Ether. Continue west, then north and west again to find a Phoenix Down. Follow the unexplored passage south, then west. Take the northern passage with lanterns. Go west to find a chest with 5000 pg. Continue north, then take a detour north-east for a chest containing Hermes Sandals. Stick to the central passageway west. In the north-west corner, head south for a chest with a X-Potion. Finally, take the southward path on the right to reach the stairs to B2.

Prison B2: From the stairs, head to the bottom-right corner and follow a secret passageway east, then north into a prison cell. Open the chest for Power Bracers. Backtrack to the stairs.

Go north from the stairs and enter the prison cell to the west. Find a secret passageway in the bottom-left corner, leading to a cell above with a Genji Helm.

Continue east from the cell above the stairs. At the end, head to the south-east corner and hug the wall opposite the lantern to find a secret passageway south, leading to a cell with a Safety Ring.

Head north to the north-east corner. Use the hidden path in the bottom-right corner to reach a cell below containing Genji Armor.

Go west along the upper corridor. Before picking up the shield, enter the left-most prison cell. Pass through the wall on the right, along the bottom, to reach the central prison cell and find an Elixir.

Inspect the shiny object in the north-west corner to obtain the Grand Marshal's Shield.