The heavy box is a quest item found during the Royal Trouble quest. It is obtained by killing the Giant sea snake encountered at the end of the quest. The box must then be taken to Queen Sigrid to receive a Letter that has to be delivered to King Vargas. The box will then stay in your inventory as an Empty box - which has no further use.
For an exhaustive list of all known sources for this item, see here
- Although the box says it is heavy, it weighs 0.01kg.
- While most items are left on the ground for 2 minutes after the death of a monster, the heavy box stays on the ground for 5 minutes after the death of the Giant Sea Snake.