You have to buy the thavnarian mist (I probably butchered the spelling on that so bad lol) from Auriana and take it to the forge near Gerolt to upgrade your relic. You'll need 900 myth to buy 3 mists. On a side note the paladin relic is unique because every other jobs relic is 1 weapon but since paladin has Curtana and Holy Shield you can upgrade them desperately. 1 Mist for Holy shield (300 myth) and 2 for the curtana (600 myth).