I think it depends on which difficulty you're playing, but it's probably in Tairon. Remember the big courtyard after the room with the twin serpent statue? You fought some soldiers there, before a door unlocked and let you continue on, way back in Chapter 4.
Anyway, if you had followed the alley around to the right instead of going down the stairs that were on your left, you'd have come across the Lunar in a jar and a gate that leads back into the twin serpent plaza.
You can go back there and claim it whenever you're next in Tairon, I believe.
When I log on to GFAQs I leave my humility at the door. - Cro Magnon Sin