An imp-in-a-box is made by catching an imp with a Magic box. It can be used by right-clicking on it and choosing "Bank Imp-in-a-box", then a window will come up where the items to be sent to the bank can be selected (be careful to not click twice quickly, because items can move after clicking on the first one). Alternatively, simply use an item on the box for it to be sent to your bank. This method is a quick way for players with 71 Hunter to quickly bank items in, for example, PvP areas. The imp will agree to take two items or stacks from the player to the bank in exchange for its freedom. However, it will refuse to do so past level 30 Wilderness, in dangerous red Clan Wars portal, or within your Player-owned House. Unlike a Beast of Burden however, there doesn't seem to be a limit on the value of the item, allowing you to bank valuable items worth hundreds of millions with the imp. The imp-in-a-box can also bank most untradable items that a familiar can't carry.
Note that when the box can be used to send two items to the bank it will be labelled Imp-in-a-box (2); when one has already been used, it will be labelled Imp-in-a-box (1) and when both have been used, it will revert to Magic box.
While you can use another imp-in-a-box to send the empty magic box home, it depends on the value of the drops you are getting whether this is worthwhile, given that magic boxes only cost 604 coins.
For large volume collection of resources that can not be used with a sign of porter, combine a beast of burden familiar with your imp boxes. Send your resources and empty imp boxes to the bank, then fill up your familiar, and your inventory and travel home. This will allow you to transport: 28 items in your inventory plus 28 items banked by the imps (with boxes banked also), plus your familiars cargo space worth of items. If you have a war tortoise, that's another 18 spaces for a total of 74 items per trip. Using a pack yak, that's another 30 items, for a grand total of 86 items per trip.
For quest items that cannot be handed to a familiar, the method above still works. Simply hand your empty magic boxes to your familiar until it is full, then ship the remaining empty boxes to your bank with imps from occupied boxes. In each case, the sums come out the same. Imps either take one item and one magic box home, or the box is handed to the familiar, and the imp carries 2 items home. 84 phoenix feathers or nightshades per trip is a great improvement on 28. The pack yak can hold all of the boxes, but 2 spaces will be left over that cannot carry quest items home.
An imp-in-a-box cannot be used to transfer any divination energy to the bank, with the exception of elder energy. If the player is carrying cursed energy or cursed memory in the Wilderness, the imp would refuse to bank anything at all. Outside of the Wilderness, the banking interface would be available but cursed energy still cannot be banked with it.
Attempting to disassemble an imp-in-a-box will yield the message "You monster! You can't disassemble a living thing."