There are 12 Stellazzio coins, plus a 13th hidden coin, to find in different areas at any point during the game, even on Disc 4. Take these coins to Treno and offer them to Queen Stella. You receive a reward for each coin you hand over to her. The prizes get better with each coin you hand over, regardless of the order in which you do so.

After you give Queen Stella all 12 Stellazzio, speak to her again. This time, she senses that something may be missing. If you count the Stellazzio cages in her chamber, you'll notice that some are still empty. Upon agreeing that something is missing, the Queen charges you with finding the hidden 13th Stellazzio. To aid in your quest, she returns all 12 Stellazzio to you, so that you can read the clues inscribed on the backs of each one.

The clues from the 12 Stellazzio are as follows: "...headed east."; "...Through the forest and over the mountain..."; "...by the river..."; "...the cape...the sun was setting into the ocean..."; "...waiting for the sun to set in the ocean..."; "...watching the sunset from the cape..."; "...opposite the direction of the sun..."; "...always looked at his shadow, until one day, he deciced to look away. He walked up a hill..."; "...dashed through the night, defying the chilly northern wind that was stinging his right cheek..."; and "...ran up a hill toward the sun and fell asleep...". The clues on the last two Stellazzio, Aquarius and Pisces, serve only to wrap up the story.

Assembling all the clues into one, you can determine that the 13th Stellazzio is up a hill above a river, beyond darkness where the sun seems to be setting, somewhere to the southeast. Thus, if you leave Treno, the city of perpetual night, and head southeast to Quan's Dwelling, you'll find the Ophiuchus in the spot where you previously found the Scorpio Stellazzio. Incidentally, the back of the Stellazzio provides the conclusion to the story inscribed on the backs of the Stellazzio.

Head back to Treno and return the 12 Stellazzio to Queen Stella, plus the Ophiuchus. In return, you receive the Hammer. Now what's the purpose of a weapon that you can't equip on anyone? If this weapon is in your inventory at the end of the game, you'll see a hidden sequence. Also, read about the Hades side quest to find out more to do with the Hammer!