Heirs of the Kings

You unlock this achievement at the same time with the holy sword achievement and all of the holy weapons can be found in the Envy Shrine left of Zarpatt. To collect them you have to fight some bosses and the way to the hardest one. It's not hard to miss, but some of them can be hard to beat. If you have a hard time, farm KHP and invest in Ability Seeds to raise your stats of Grant + upgrade his Final Strike skill.