The One King is a major antagonist in Suikoden Tierkreis.
His goal is to merge all worlds within the Infinity into his own worlds where the inhabitants' best day in their lives repeat forever, thus creating a future that is always predetermined.
It is unknown when the One King appeared, but he could be assumed to have first shown up at least 8 decades before the start of the game. As said by him during the game's main ending, the One King is created when the Tenkai Star of a world is defeated by The One King, thus he has no future or past, only conflict.
The Order of the One True Way in many worlds worshipped the One King and awaited his arrival. He would arrive when 12 worlds were "fused" with that world, which usually meant a century to wait. However, many leaders of the Order learned of a ritual that would allow the One King to arrive much quicker. Because of this, the worlds within the Infinity began to diminish quickly, giving the One King an incredible amount of power.
The One King is shown to be very silent, even in combat. He is completely emotionless, even though he is made up of many Tenkai Stars. Despite how he destroys worlds, it is shown that he does this against his will.
The One King’s name is only heard vaguely before. Cougar was fighting the One King before he perished, and Fredegund had heard Beardsley speak of that name. Once Sieg learns the name however, he learns that Atrie and Marica? are currently fighting to prevent the One King from appearing in their own worlds as well. Diadora also recalls that the One King destroyed her original world as well.
When Sophia joins, she tells Sieg that the One King was originally supposed to come to the current world and end it after five fusions. Valfred was bent on summoning The One King, even performing a ritual to speed up his arrival. Even though the ritual sped up the arrival, it is not until the end of the game that the One King finally arrived.
Sieg and The Company attacked the One King when he arrived, but did no damage whatsoever. However, the Chronicle of the Shining Legacy glowed, and The Company attacked the One King once again. Sieg noticed that they were doing damage, but very little. Because of their loss, The Company was temporarily trapped within the One King's World.
After his victory, The One King set up 12 pillars of light that protected Cynas. Within Cynas, he set up a giant stone statue of himself, which acted as his base of operations.
After Sieg and The Company escaped the One King's World, they learned of the Chronicles' power and infilitrated the One King's Stone Statue.
Once The Company made it to the One King's Room, Sieg activated the power of the 12 Chronicles, allowing him and his teammates to actually damage the One King. After his defeat, The One King changed into two different forms, which were fought by all the Starbearers of the Infinity.
Inside the tower, Sieg notices a Stone Tablet of Promise, just like the one in his castle, with the only notable exception being that all the names on the tablet bar but one were scorched off and illegible. Piecing things together from things told to him by Atrie and Zenoa, Sieg reason that the One King was once a man, who united all of the energy of the Starbearers into one vessel.
After both forms were defeated by the Starbearers, The One King appeared with Sieg in front of the corrupted Tablet of Promise. He explains the cycle of which The One King was created from, and gives Sieg the True Chronicle. After that, The One King completely vanishes from the Infinity.
His defeat however, prevented the use of Gateways to travel to other worlds within the Infinity. This made it harder for Wanderers to do business, and meant that Sieg could not visit the Marica ? or Atrie.