One fateful day, Stampy was walking through a forest with his dog, Greg. He came across a pond which had black sludge in it, taking the place of water.
Stampy walked up to it, curious about the strange matter. Greg stepped towards it and fell in, leaving Stampy to dive in after him. Accidentally they both drank the sludge.
A few days later, Greg turned on him and attacked. This left Stampy with bite marks all over his body, which eventually turned black, as more of the sludge seeped into him.
As StampyLongKnife, he entered The End, and quickly realized that he was unable to live in The End. He wandered back to the forest where he had first encountered the mysterious black sludge. He took over the forest, forcing it's inhabitants out and killing anyone who dared enter.
Stampy slowly became more cat-like, rarely walking upwards. He grew knife-like spines along his back, which became his namesake. Enderlox once entered the forest, but Stampy drove him out, his demonic dog minions severely injuring Enderlox's wings, leaving him unable to fly for a week.