Phantom Sword

The Phantom Sword is the only weapon that can harm Bellum and Phantoms in. It is composed of the Phantom Hourglass and a blade forged from three types of Pure Metals: Crimsonine, Azurine, and Aquanine.

Overview
After rescuing Tetra from the Ghost Ship, Link is told by Oshus to find the three kinds of Pure Metals and take them to the legendary blacksmith Zauz to forge the Phantom Sword in order to defeat Bellum. After Zauz forges the sword with the three Pure Metals, Link must complete the blade by returning to Oshus, who will fuse the Phantom Hourglass with the blade itself. By doing so, the Phantom Sword gains the ability to control time for a short period.

The Phantom Sword is the only weapon capable of slaying the Phantoms that patrol the Temple of the Ocean King, and thus, the only weapon capable of defeating Bellum. Since the Phantom Hourglass is incorporated into the hilt, it makes the wielder immune to the life-draining curse of the Temple. In combination with a Phantom Sphere, it can stop time temporarily. Link uses this ability in his battle with Bellum, using it to freeze time so he can get behind Bellum and attack his eye.

History
When Link goes to talk to Zauz about the Phantom Sword, the blacksmith explains that his ancestors defended the Ocean King in an age when great evil lurked the sea, and in order to combat those monsters, his people would forge their own weapons. One of these weapons was the Phantom Sword itself, and Zauz even mentions that he had one, although he lost it at some point. Thus, it is implied that Zauz's people forged many Phantom Swords for battle, and that perhaps the Phantom Sword is not the only weapon that can vanquish Bellum and the Phantoms.

Hyrule Warriors Series
In the Series, The Phantom Sword is the second tier weapon of the Light Sword for Toon Link, paired up with the Shield of Antiquity.

Trivia

 * The Phantom Sword resembles the Master Sword in many ways - the major differences between them are the symbol of the Ocean King appearing in place of the Triforce emblem on the blade as well as the crossguard featuring a different design, in which the Master Sword's yellow jewel is replaced by the Phantom Hourglass.
 * The music that plays when Link receives the Phantom Sword is also the same as that of the Master Sword.