Strongbox

Strongboxes are a map mechanic. The exile may encounter chests with varying rarity, including unique. These chests may be crafted upon with currency items. When the chest is opened, a dark mist will surround the character, and monsters will approach. After they are slain, the chest will open and drop its rewards.

This mechanic is extremely similar to a Path of Exile 1 mechanic of the same name.