Dragon Ball AF is a blanket term for a number of fan-made series concepts imagining a potential sequel to Dragon Ball Z and Dragon Ball GT. One of the most successful of these was a webcomic by the Japanese fan artist Toyble — who would later join the Dragon Ball franchise proper as Toyotarō and work on the manga version of Dragon Ball Super — who worked to incorporate every rumor about the proposed (non-existent) sequel, including the prospect of an evil son of Goku, whose arrival was the core storyline for Toyble's fan-manga.
Zaiko, otherwise known as Xicor, arrived with a genuinely unexpected backstory and powers specifically tied to a transformation unlikely to make it to the core Dragon Ball Super storyline. But it's still worth noting who this fan-created character is and just how powerful he (briefly) became.
Who Is Xicor?
Many versions of Dragon Ball AF reintroduced Kayoshin, the Western Supreme Kai who was seemingly slaughtered by Majin Buu in Dragon Ball Z. Here, she's re-imagined as a secret villain and the mother of Frieza, who'd united with King Cold in the hopes of producing the galaxy's most powerful fighter. After Frieza's defeat and her disappearance, Kayoshin took a DNA sample from Goku and combined it with her own, creating the villainous Zaiko — otherwise known as Xicor. The two traveled to Earth, where Xicor quickly proved to be very powerful. He was more than capable of keeping up with the likes of Vegeta and Gohan, who in this continuity has reached Super Saiyan 4 and Super Saiyan 5.
The only reason Xicor wasn't able to wipe out the heroes was because of Kibito Kai, an ally of Gohan's from the Buu Saga and Kayoshin's brother. Using the Z-Sword, he's able to seal away Xicor and contain his power, sacrificing himself to kill Kayoshin as well. However, Xicor's sheer power only gives the heroes a brief amount of time — roughly a month — to prepare before he breaks free of his prison. The heroes endeavor to revive Goku, perhaps the only person who could stop his genetic offspring, but he's been trapped in a different dimension since the conclusion of Dragon Ball GT. The need is so great, the heroes agree to revive Broly to reach the seven Saiyan requirement for reviving Goku.
A cruel and mocking villain, Xicor became the chief antagonist of Dragon Ball AF, proving surprisingly similar to a more powerful version of Cell from Dragon Ball Z. He's quickly set up as a massively powerful force and embodies many of the worst elements of Saiyans. He's a battle-eager and ruthless fighter who enjoys toying with his opponents, and he's powerful enough to do so to major warriors like Vegeta. But unlike Broly — who was likewise an absurdly powerful Saiyan but far more brutish and uncontrollable - Xicor is a maniacal figure. He lulls Vegeta into a consistently false sense of security before mercilessly beating him and leaving him a wounded mess. He insults his enemies, and laughs when they're on the verge of death.
His sheer power makes him a nearly unstoppable threat, and his casually cruel nature could be seen as a stark contrast to his biological father's friendly and simple attitude. Due to Xicor's fan-created nature, there isn't really a single, strict version of him, or even a specific ending for the character —an eventual battle between Xicor and Goku was teased for Toyble's version, but Toyble eventually hastily ended his fan-series on a sweet note, with Roshi in the future, the last surviving member of the original Dragon Ball heroes, telling the story to Goku Jr. and Vegeta Jr. and dying peacefully before he can finish it.
It's unlikely that someone like Xicor will appear in the canon Dragon Ball franchise, especially with his specific direct connection to Goku. It's a very weird and dark place for a character to begin. Instead, Dragon Ball Super introduced Goku Black, a corrupted and possessed version of the hero who proved to be a more memorable and complex version of a similar idea. They even both derive from the Kais, in a fun coincidence. With Super Saiyan 4 also relegated to Dragon Ball Heroes instead of the core canon, it's unlikely someone whose major claim to fame is achieving Super Saiyan 5 would appear. But it's worth always remembering the fan community's evil Goku and just how strong he was, reaching the ridiculous levels that only fans could imagine.