Hero Loot is required to promote a Hero, and you must equip a set of loot to do so. The loot can be obtained by hunting monsters. Each monster drops different kinds of loot. You can check where the loot is dropped by clicking on the loot icon, and clicking “Where to Find” to see if there is currently a monster nearby that will drop that specific loot.
If the specific monster hasn’t appeared yet or is located far away from your current location, it will say “It Cannot be Found”.