To finish this quest you have to obtain the ring by combining the parts, which drop from the monsters at Cadry Ruins, using the https://bdocodex.com/th/puzzle/92/. Buying the ring on the market will not work. There is also a way to craft the ring in Altinova, using the Ring of Cadry Guardian design. However I didn't tested if it'll count towards the completion of this quest or not.