First you talk to the npc named Kozumi. than you pick the quest called Retrieve the Stolen Relics lv 50.
then you can go inside of Caronia?s tomb or you can go to serpents swamp which is located here:
You have kill serpenter caliph(lv55) they randomly give you the relic so its gonna take awhile
After that you report back to Kozumi
this quest will be hard if you dont have good mdef, so most people should wait till there higher lv to do this