He was formed after TrueMU was killed by the now-Canadian Ender Deadlox, or Enderlox. The body of Wither is like TrueMU's body and the two small heads are like TrueMU's Helmet but the middle head is like Herobrine's.
It all started one day when TrueMU was fighting the wither boss sent by TheFearRaiser. He successfully killed it and got his prize; The Nether Star. He was so glad and excited to tell his friends (Deadlox and Sky) about this but he started to suspect there was someone watching him. From out of no where, Enderlox came. TrueMU tried to kill him but Enderlox was really agile and TrueMU quickly became confused. From behind, Enderlox sneaks up on TrueMU graving him from the neck. Helpless, TrueMU tries to cry out for help, but it's useless. Enderlox tightens his grip even more until TrueMU stops struggling and dies. Enderlox lets him go and leaves the dead TrueMU on the ground. TrueMU's body was later found and buried. Both Sky and Deadlox heard about this.
Just a day later after TrueMU's death, Enderlox takes the Nether Star that TrueMU got when he killed the wither boss and later went to TrueMU's grave. There, using the power of the star, he revived TrueMU but something was different about TrueMU. Leaving TrueMU behind, he disappeared and was never seen again. TrueMU was confused and exhausted, so he got shelter and slept. The next morning, TrueMU had felt that he had to take off his helmet and saw his new white glowing eyes like Herobrine's. TrueMU, now afraid, started to wonder what that Enderloxdid to him. But white eyes were only the start.
Each day, TrueMU would have a new change. After his white eyes, he lost his arms, had 2 heads, lost his legs and had a tail and ribs and last he was able to fly. TrueMU didn't care of this now and wanted to have fun. He broke many towns and villages with his new self. But after all this, he finally remembers about his friends (Sky and Deadlox) but didn't know if they'd like the new TrueMU. To his surprise, they too were monsters. He joined his new friends (Skybrine from Dublin, Ireland, and Enderlox from Toronto, Canada).
Even though he is now a Wither-esque mob living in the End, or more like Toronto, he has the ability to turn into human form, which he uses to trick and kill people.