As you venture through the Regalith Grand Cathedral dungeon in , you'll stumble upon a locked chest atop a short staircase in the Servants' Chambers. While most haven't required much work to open before this point, you'll need to find two keys throughout the church to get your hands on the chest's contents. This starts with following the soldiers who run off when you first enter the area.
The Cathedral is home to various enemies to take down during your investigation, namely stray Morneto variants. It doesn't take much to beat these undead foes, but it's a good idea to heal up and cure common status ailments, as a much stronger Homo Oppo enemy lies just around the corner. Their defeat will lead you to the keys you need to move forward.
The first step in opening the strange chest is locating the Guest Room Key. This won't take you too far from the room in which you first found the chest; head east from there and follow the corridor. You'll see an entryway blocked off with mounds of rubble down the hall and a Homo Oppo enemy wandering just beyond this point. Take it down with Hulkenberg's Perfect Punch and powerful Archetypes in to get it out of the way.
Metaphor: ReFantazio has the Bonds system similar to Social Links from Persona. These extend to both party members and NPCs, granting 14 in total.
In a room on the right-hand side of the hallway, you'll find a gravely wounded monk by the wall. Speak with the Dying Monk, and he'll inform you that the soldier who stabbed him passed away nearby. To find him, smash the barrels between the two bookshelves to reveal a crawlspace, then head through to the next room.
Grab the Ailment Absolution inside the first room's doorway before leaving. This item will come in handy should you need to cure any status ailments outside of battle.
You'll find the soldier on the floor straight ahead in the next room. Gallica notes that he has a key hanging around his waist when you interact with him. This is the Guest Room Key you need.
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