Necklace | Kingdom Hearts

The Kingdom Key (キングダムチェーン) is the most recognizable and the first Keyblade to appear in the Kingdom Hearts series, and is the form used in almost all depictions of the weapon. It has appeared somewhere within every game in the series.
The Kingdom Key draws out the true power of Sora's Keyblade, even though it is not the strongest Keychain. As such, it is the true form of his Keyblade, which means that it is also the default form of Roxas's and Xion's Keyblades. The Kingdom Key is not wielded by Sora exclusively; Riku, Roxas, and Xion have also used it, and King Mickey is often seen wielding the Kingdom Key D, a color-inverted version of the weapon.