This quest is initiated by examining a Boar's Head on a wall at -11.95, 0, 40.51 . You can pop out the eye of this boar's head and find a new home for it.
The new home would be a boar head that is missing an eye at 29.66, -0.02, -33.03 in the Dining Room.
After replacing the eye in the Boar's head you will get text saying you receive a Barracks key. You should now be able to click on the box in the barracks ( 18.09, 0, 31.64 ) to initiate the quest, "The Red Marble".
Aziide had these tips for finding the boar's heads:
To find the first boar's head, follow the hallway like you're going to the chapel. Run past the chapel and to the end of the hall. The boar's head is above a doorway that leads to a deadend of freaky stairs.
The second boar's head is practically opposite of it. Turn left from the courtyard as if you're going to the game-room. Instead of turning left down the hallway, swing a right. The first door you come to will be the dining room. Kill the Thexian guards in there and head to the back of the room. The boar's head is up on the wall.