Initially, talk to the woman in the house north of the Treno Combination Shop. She will tell you all about the thirteen Stellazzio that are dispersed across the world. Naturally, if you can find all these, she’ll give you a reward on their return! This quest is required if you want to see the game’s alternate ending, and is thus very worth while!

Here are the zodiac / Stellazzio locations:

Aries: Dali; In the windmill all the way to be back (not downstairs)
Cancer: Burmecia; behind the overturned cart
Scorpio: Quan Cave spring
Gemini: The fountain at the first screen after you enter Treno; if you toss coins thirteen times into the fountain.
Taurus: Treno; behind the item shop.
Virgo: Black Mage Village Inn; amongst the beds
Libra: Madain Sari fountain
Leo: Alexandria (after raid); the Left Tower near Neptune Statue
Sagittarius: Linblum; in the left side of the 3d screen up of the
Commercial Square.
Capricorn: Dargelo Library; right hand side
Aquarius: Ipsen Heritage; right hand pillar at entrance
Pisces: Treasure chest inside Invincible.
13th Stellazzio: Return to the Quan Cave that’s east of Treno after collecting and returning all 12 coins.

You’re now ready to collect your reward!