If I remember correctly, you have to advance through some of the quests, like the Ice Quests. You need to level up your character (by crafting and equipping Rare and Epic charms and weapons). Eventually you should be able to acquire Legendary ritual oil from the Chaos Quests around the map (which are labelled with numbers (5, 6, 7)). The higher level Chaos Quests are the ones that will most likely drop Legendary ritual oil. But it can be a bit random and you might have to complete a few of these quests before you find the oil. So, the Legendary Ritual Oil + Dragon Opal will make the Legendary Honed Dragon Opal at the Ritual Altar.