This page details how to complete the Roman Excavation at Diodurum Ruins in Melunois. The excavation forms part of the Hidden quest, but the mystery will complete once you’ve obtained the key.
Roman Excavation: How to Find the Key at Diodurum Ruins
When you reach the Diodurum Ruins (located west of Paris) and use your raven to scout it, you’ll see a breakable wooden barrier near the centre of the ruins. You won’t actually find an entrance through here, so don’t bother exploring it. Instead, go to the pond located east of the ruins, just beneath the cliff, and enter through the cave on the northern side of the pond. As you swim through you’ll eventually reach a point where you’ll need to dive under the water to proceed. You can then climb up to reach another door which can only be opened with your Hidden Blade.
Interact with the door to open it, then jump down the hole on the other side, into the water below. Swim northward, then west, then south, until you’re able to resurface in a room with some fire pots and a breakable wall. Use one of the fire pots to destroy the wall, then proceed into the room with the Book of Knowledge, which provides the Golden Flame ability, and the Diodurum Key on the table at the northern wall.
Once you have both the Book of Knowledge and the key, you can exit the Diodurum Ruins from the same way you entered, or by moving the barricades beyond the door to the south.