The Ancient Treasure Chest is an Age-Old chest that is locked and spawns only in The Room. It doesn't work like any other chest in the game. It is the only thing inside The Room, aside from the portal that takes you to your spawn point in the overworld. It contains the Ancient Clock, which is the item that makes the Time-Machine work. It is one of the only blocks that has an animation of it moving.
The Ancient Treasure Chest is an old black version of a chest. It also has a strange symbol on the front of it, that looks like Anu, the boss of the mod.
It can only be used if you right-click on it with the Ancient Key, which will slowly open the chest. Then, an Ancient Clock will pop out and it will be yours to take.