The imp cache is a creature bank in Heroes III and Heroes IV.
Heroes III[]
Heroes III imp cache
In Heroes III, the hero fights imps and familiars and is rewarded with gold and mercury.
Heroes IV[]
Heroes IV imp cache
"Some nasty, trouble-making imps have made this place their home. Much of what they collect is trash, but there's likely to be gold and sulfur among the filth."
In Heroes IV, the hero fights imps and is rewarded with gold and sulfur.