Nethicite is a powerful source of magickal energy. A subtype of magicite, nethicite absorbs copious amounts of Mist and can release it as magickal energy. Historically most nethicite was created by the Occuria ("deifacted" nethicite), but Archadia has discovered how to create artificial "manufacted" nethicite.
Deifacted nethicite is too strong to be controlled properly, and dangerous when it cannot be. Manufacted nethicite is weaker, but safer and easier to use. Nethicite can disrupt magickal energy and machines that rely on it with its presence, and can absorb magickal attacks. Charged with Mist, it can create skystones resistant to jagd, be used to summon Espers, and can even be implanted in a person's body to augment their physical strength and magickal abilities, although such use of nethicite damages one's sanity and eventually causes the host's body to break down into Mist.
Deifacted nethicite was created by the Occuria in the form of the Sun-Cryst. They sought to give nethicite first to the garif, but they could not or would not use it as the Occuria demanded, so they turned their attention to King Raithwall of Dalmasca instead. They granted Raithwall the Sword of Kings, which could cut nethicite, and Raithwall took three shards from the Sun-Cryst. He went on to conquer much of the nearby territories and founded the Galtean Alliance, of which Dalmasca, Nabradia, Rozarria, and Archadia were members. Raithwall was crowned the first Dynast King, a uniter of Ivalice.
When Raithwall died the three shards he had cut were split up. The Midlight Shard was given to what would become House Nabradia, the Dusk Shard was kept in Dalmasca—hidden in the Royal Palace of Rabanastre—and the third piece, the Dawn Shard, was buried with Raithwall in his tomb, a secret kept by the Dalmascan royal family. The peace he had brought to Ivalice endured for hundreds of years.
One of the Occuria, Venat, believed that using gifts of power to control the factions of Ivalice was wrong and sought to break their hold over Ivalice and free her people from Occurian control. Seven years ago an Archadian expedition led by Cidolfus Demen Bunansa ventured into Giruvegan, where the seat of the Occuria rested. Venat made contact with Cid and the two entered into an alliance. Cid returned to Archadia with Venat and, guided by the Occuria, synthesized artificial nethicite from ordinary magicite mined at the Lhusu Mines. Over the next several years Archadia's power grew with nethicite empowering their armies, and were even able to develop skystones that let their airships fly over jagd, something impossible with normal skystones.