The Lead box is an octagonal wooden box where the Goblin had been trapped inside for more than 50 years. According to Gaius' grimoire, Goblins can only be held captive in a box lined with lead. Merlin found one such box when he was looking for the Bestiary and accidentally triggered a secret door which lead to a hidden room where the box was kept. Soon after that he opened the box and accidentally released the Goblin, who had been imprisoned in the box for over 50 years and immediately started causing trouble after being let out.
Sometime after that the Goblin entered Gaius' body. It took some time for Merlin to force him out of Gaius, and when he did, the Goblin was once again put back inside the box. Later the box was shown to Uther. Gaius suggested that it be kept where no one could ever open it and Uther ordered that the box would be placed in the vaults (Goblin's Gold).