The complete Fusion chain to Divine Staff uses just 9 ingredients, 2 of which provide no bonus as they only drop from chests. The 7 remaining are the Necromonicon and Earth Emblem of the final fusion, and 5 Darkness Crystals. Hence a Divine Staff +15 requires at least a True Crystal Rod +7, though higher is recommended.
The Divine Staff's name in the Japanese version of Recettear is "Natural Disaster" (天変地異).
Similar to its role in Recettear, it is also the ultimate magical staff in Chantelise, EasyGameStation's previous (and CarpeFulgur's next) game.
Due to its base price exceeding his budget, this is the only item on Caillou's likes list that he cannot select from a counter; consequently the only way to sell it to him at base price is as part of an order for weapons.
As orders do not count for an item's record sale value, the only way to get its highest sold price to match or exceed its base price is to sell one to someone other than Caillou.