Kezeroth is an incredibly powerful entity who is feared for being known as the "World Ender". He plans to live up to his name by freeing the ancient beast Quetzal from its prison in a frozen comet passing by Lore every 26,000 years. His first attempt to do so failed at the hands of the legendary hero Elim, who redirected the beam of energy that would've brought Quetzal itself down to Lore to end it away from it, so the ChronoCorruptors froze him in time for another 26,000 years so they could take the liberty of fixing the temple for him and give him another chance to free the beast Quetzal. Athon, a Chronomancer who was watching Elim stop Kezeroth's plans, saw the ChronoCorruptors do their job, and went to the players for help in defeating Kezeroth again and preventing the end of all things.
The ChronoCorruptors unsealed Kezeroth anyway and he was then able to free Quetzal from its prison. Luckily, the players and Athon retrieved the Sword of Hope from the bottom of the FrostDeep Dungeon below the temple and teleported out before the comet hit the temple. After the players defeated Quetzal, Kezeroth, having seen the outcome of the battle, angrily finished off Quetzal himself as punishment for failing to bring about the end of all things before breaking the sword, and he departed for Ice Rise Keep telling the players to stay out of his way.
The players decided not to heed his words anyway and they stormed the keep to confront Kezeroth. As the players dealt as much damage to him as possible, it became apparent that Kezeroth was proving to be unstoppable until Athon got an idea. The Chronomancer gave his life to save Lore from Kezeroth by using up all of his energy to freeze Kezeroth in time for another 26,000 years or even longer.
It is revealed during the events of the Frostspawn invasion that Karok the Fallen is, in fact, Kezeroth's brother.
Two years later after that revelation, Karok, having realized over those years that he wasn't able to use the Queen of Monsters to get what he wanted, decided to revive Kezeroth from his imprisonment. He arrived at Ice Rise Keep on IceWind Pass where Kezeroth's icy prison was currently resting and succeeded in freeing him, hoping to convince him to work with him in his quest for world domination. Unfortunately, Kezeroth, who had somehow lost his memory of what happened four years prior to his re-freedom, was furious when his brother told him that it had only been that exact amount of years. He again believed that Quetzal's comet would only come 26,000 years later and that his brother had robbed him of his chance to finish his work. Karok attempted to remind him that Quetzal was already dead and that he destroyed the beast himself when it failed to bring about the end of the world for him, but due to his lack of memory, Kezeroth would have none of it, saying that he wasn't interested in conquering Lore and instead was destined to destroy it. Kezeroth vowed to kill his own brother before allowing him to control him, and from there, a fierce battle between the brothers started. Abel, Syrrus, and the players, who had been watching this recent argument between the brothers when they arrived, decided to use this falling-out between them as a chance to use one brother to take down the other.
Kezeroth destroyed many of Karok's Frostspawn minions and overpowered his own brother. Before he could finish him off, though, Karok binded him to chains bending him further to his will, to the shock of their spectators. They summoned the Soricomorpha, which they referred to as the Seismole, to attack the heroes as they made their getaway.
Later, the players and Syrrus were able to retrieve the Crystal of Glacera, an artifact that would have enough power to defeat Karok and Kezeroth, just as the Frostspawn arrived to attack them. After the players refused to give the crystal to Karok, Kezeroth appeared and fought them, but thanks to the Crystal's power, they managed to defeat him and freeze him in place. It is possible that when Karok convinced them to form a truce with him, they most likely would have allied with Kezeroth as well as a result of this newfound truce.