A void key is a magical item on Cradle that provides access to an interdimensional space separate from the main world, known as a void space (though "void key" is often informally used to refer to the spaces as well). Void keys are quite rare, even for Lords; creating them requires the powers of at least a Sage. Relatedly, Sages skilled in spatial manipulation can open their own private void spaces directly using their own authority, without needing a separate artifact.
The physical part of a void key can literally look like a tiny key, although only one has been explicitly described. Void keys open when the physical key is provided with a flow of madra. Only a tiny amount of madra is required, but keeping a key open does require a certain level of concentration.
Void spaces can be of widely varying sizes, ranging at least from a picnic basket to a large cloudship. Void keys are categorized into different grades, presumably based on their size, with at least low-grade, high-grade, and top-grade keys in use.
Living beings can stay inside closed void spaces with no immediate problem, but most spaces are not set up to support survival and would be difficult to stay in for long due to their limited volume of air. Also, someone's presence inside a void space somehow interferes with the ability to use spatial manipulation to move the void key in the outside world. Spirits, though, are much "lighter" in some sense and don't cause the same problem.
The Dawn Sky Palace is a void key that is specially set up with a house and the ability to support life inside it. Some Akura void keys are filled with shadow madra, preventing anyone from seeing inside them.