Note: Despite not knowing the item's name, we shall refer to the item as the "Key of the Tower" given the current information.
The Key of the Tower, is a 'key' which opens the door of the Tower.
On the day Zahard declared himself the King of the Tower he decided to stop climbing, thus hiding the key that would open the door leading to the higher Floors so that no one could go beyond the 134th Floor.
Apparently, Zahard divided the key into two parts. He gave the first half of the key to the blacksmith Ashul Edwaru who melted it and turned it into 13 separate weapons, so that the key would be shared between Zahard and his 12 companions. Eventually, these weapons were instead given to the Princesses, who were subsequently made to fight amongst themselves with them. The other half of the key was melted into several crimson rings displaying the crest of Zahard. These rings were handed to children who inherited the blood of Zahard, whose existence was meant to be kept hidden alongside the rings. Two of these children, Ja Wangnan and Karaka, have been revealed.
It is said that when all of the these 'princes' of Zahard are able to climb the Tower, the door of the Tower will open once again. Because of this, many speculate who will be the real "Prince of Zahard" that will take the Tower’s last test.