Simply called Tai is a clone created by Datamon, who some how managed to survive his last skirmish with Etemon.
The clone of Tai was created by Datamon, an old enemy of the original DigiDestined. Through some incredible means, he survived his last battle with Etemon. Datamon intended to get revenge on the one DigiDestined who foiled his plan and it was Tai. Like his previous plan, he intended to create a clone of Tai, like Datamon did with Sora. However he supplied the clone with his own Digivice and even a partner Digimon which was a BlackAgumon. Datamon did so by luring Tai with a phony distress message and he was shocked to find Datamon alive and well. Tai and Agumon were both taken prisoner and soon Datamon began scanning and cloning his two captives.
His cloning procedure was an immense success and so he wished to test his clone by having it lure the rest of the DigiDestined back to the Digital World. Tai's clone was designed to be just like the original, but with extreme loyalties to Datamon only. A battle between the clone and DigiDestined went underway. BlackAgumon had abilities similar to the original and could even Digivolve into a champion form like the original. Which was to everyone's shock and horror. The battle continued on, but soon by accident, the clone of Tai was hit by a stray attack by one of the Digimon. He was soon flung against a Control Spire and his data soon became corrupted. His body absorbed the evil influence of the spire and it changed him drastically.
For one: the colors of his clothes and hair changed, but so did his personality. The Control Spire made him a fully sentient being, so he then no longer wanted to be Datamon's puppet. Datamon proved to be angry at his creation's betrayal, but the clone did not care. He completely rebelled against Datamon by absorbing his data the same way he absorbed the Control Spire. The clone proved to be a force to be reckoned with because he also double crossed the partner that was given to him. He absorbed BlackAgumon's data too and he grew even more powerful by the second.
The clone then proves to be a major foe for the DigiDestined as he can absorb Digimon and use their powers for his own. In time he was eventually defeated by the DigiDestined and so he vowed to not only destroy the Digital World, but the real world too.
The clone is a very twisted and evil being who hates humans and Digimon alike. Being merely a clone made of data to look like a human, he feels that he belongs in neither the Digital World or human world, so he wants to destroy them both.