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The Urn of Gaia is located in the Colossus level in God of War 2. To get it, you will have to past a moving block puzzle, so make sure that you follow thid God of War II cheats guide to the letter:
You will encounter a series of rooms and doors you will need to open. Look for a statue on top of a moving block. Destroy it and move the block into the next room which will have a series of doors on all sides.
Continue to push the block into the room on the far right. Push the block until the end of the next hall. Now you will see the Urn of Gaia just above you. Climb on top of the block to reach the Urn and two blood chests.