Arcanium is used to trade with Captain Merik and other Dungeon of Arcana NPCs. It is gained by placing Arcana Ore in the top slot and Charged Extractors in the bottom slot (both of which can be found in the dungeon) into an Arcanium Extractor. The Charged Extractors act like coal in a furnace. (But the Charged Extractors only create enough power for 4 Arcanium.) You can obtain Charged Extractors by trading 16 Collector Fragments obtained in various quantities from killing mobs in the Dungeons Of Arcana.