The Sheathe of Iwaku, also known as the Tower of Iwaku, is the sheathe that is designed to contain the Sword of Iwaku. It is also the only item that can never be cut by Sword of Iwaku inside the world of Iwaku.
It also has the ability to change its size in any dimension, becoming as small as a pin or as large as a gigantic, stone fortress. This was made because the Sword of Iwaku changed form depending on its user, and thus the sheathe had to be able to accommodate whatever form it took.
The sheathe is also a potent item on its own, as the size change can be done on a whim, and it occurs in an instant. Paorou-sama used this to great effect during the Admin War even after taking the Sword of Iwaku.
It was taken by Porg soon after.