(Gaming term) An item that is vital for a quest or weapon upgrade, which is actually common in some areas or after a certain level has been reached. Used by developers to prevent the quest or weapon upgrade being completed before an area is unlocked or the characters level up.
Traditionally, hercudromes are some variety of insect, although plants have also been known to fill the role.
Hmm... what else do I need... Ice cabbages? But I haven't been to anywhere icy yet! Damn, I should have known that there would be a hercudrome...