Dynast King is a title used in the game Final Fantasy XII, given to people the Occuria have chosen to set the course of history straight. The Occuria commonly refer to the person they have chosen as the Dynast King as "Saint".
When the Occuria decide that Ivalice's history has become too wayward, they select an individual to do their bidding: guiding history along a path they see fit. To this end, they choose a person of great power, usually a king, and give them an item of even greater power, the so-called "stone of the gods", nethicite. With this nethicite, the Dynast King will unite Ivalice, and do what the Occuria bid, usually without the awareness of their manipulation.
King Raithwall was the Occuria's first Dynast King. Upon his anointment, the Occuria gave Raithwall the Sword of Kings. With this, he went to the Pharos and cut off three pieces of the Sun-Cryst; the Dawn Shard, the Midlight Shard, and the Dusk Shard. With these three shards of nethicite, he managed to ally all the great nations of the world.
This period was called the Galtean Alliance, and during this period, many great kingdoms and empires rose. He then gave two of the nethicite shards away, to the Kingdoms of Dalmasca and Nabradia, and kept the last shard with him, until his death. Ivalice was united, and war was no more, so the Occuria were content.
Seven centuries later, war had broken out between the Archadian Empire and the Rozarrian Empire. Many smaller Kingdoms had fallen in this war, and countless people were losing their lives. Furthermore, the rogue Occuria Venat had taught the Archadian Empire the means to create their own nethicite. Enraged by these iconoclastic turn of events, the Occuria decide to christen a new Dynast King to set things straight once again. They choose Princess Ashe, the direct descendant of King Raithwall.
However, in order to do so, the Occurian king Gerun tricks the princess by using an image of her late husband, Lord Rasler. Using this image, it leads her everywhere they need her to go and prevent any unwanted actions, like the Dawn Shard's destruction.
When Ashe reaches the Occuria at Giruvegan, Gerun gives her a new sword, the Treaty-Blade, with which she would carve new shards of nethicite off the Sun-Cryst and use it to destroy their common enemy to straighten history's weave. Reluctant, Ashe goes to the Pharos at Ridorana and reaches the Sun-Cryst. However, she eventually realizes mankind never needed the nethicite, that it was always working together that ensured a bright future, so she decides to destroy the Sun-Cryst.
As a final resort, Gerun appears as Rasler and begs Ashe to reconsider. She sees through his guise, and strikes the false image down. Finally, the sky pirate Reddas takes the Sword of Kings and destroys the Sun-Cryst, sacrificing himself in the process.
While Ashe and Raithwall were both chosen by the Occuria, Vayne's claim to the title of Dynast King was completely by his own power. Judge Bergan, Vayne's loyal follower, proclaims Raithwall to be just a pretender of the title. Since the original Dynast King was simply a puppet of the Occuria, he could not be called the true leader of Ivalice.
Vayne saw his ascent to throne of Dynast King as the only way for Ivalice to know true order. However, Vayne's rise to power is cut short when he is killed by his rival Ashe, her party and Judge Gabranth at the Battle above Rabanastre.
Ultimately, neither Ashe nor Vayne become Dynast King. After Vayne's death and Ashe's destruction of the Sun-Cryst, the rule of Dynast Kings is over and the Occuria leave world affairs to the hands of man.
The Dynast King, Dorgalua Oberyth, is mentioned in the backstory of the Tactics Ogre: Let Us Cling Together remake for PlayStation Portable. Dorgalua's unification of the Valerian Isles echoes Raithwall's unification of Ivalice.