This page of the Resident Evil Village guide contains a description of the Maroon Eye puzzle. You will solve this puzzle during the first visit to Dimitrescu Castle, which will allow Ethan to unlock a passage to the rooms on higher floors.
Finding Maroon Eye Ring
Finding the ring is part of the main plot. During the initial exploration of the castle, Ethan will be captured by vampires and "interrogated" by Lady Dimitrescu. Once freed in the Bedchamber room, you must interact with the fireplace since the main exit from the chamber is blocked.
Ethan must get through a narrow corridor. There's a statue at its end, the examination of which will lead to obtaining the Maroon Eye Ring and unlocking the exit.
Getting Maroon Eye
The next part of the puzzle may cause you some trouble, as the ring needs to be viewed in the inventory. You'll find it in the Key Items tab.
While examining the ring, focus on the chestnut eye. Ethan will pull Maroon Eye out of the ring. It will appear as a new object in the Key Items tab.
Using the Eye on the Prioress Door
You can now reach the south door located upstairs in the Main Hall. The main character can place the eye in the Prioress Door, specifically in one of Mother Superior's eye sockets. This will unlock access to new rooms in the castle.