The "Before" Story...
Over the years King Bowser came up with many plots to kidnap Princess peach, and to defeat his archrival, the popular Super Mario. Ever since he was a young koopa child he tried to crush Mario and his friends, the Yoshis, but as time went on, and the children grew into adults, the chances of success for the evil koopa king grew as well; because with each defeat, and each retreat, Bowser grew smarter, learning from his mistakes, until finally a faithful Magikoopa named Kammy Koopa, found a perfect plan for her master; one that would ensure success, and defeat over Mario by using the powerful Star Rod from Star Haven to grant Bowser the gift of Invincibility.
And Mario was defeated... that is, for a while. With the help of the seven Star Spirits, Mario found a loophole in Kammy's perfect plan, and came back to take out Bowser once again.
Bowser went into hiding for many years. The economy and prosperity of the Mushroom Kingdom was allowed to grow, and Mario became king. Then one day, out of nowhere, a volcanic eruption changed the face of Yoshi's Island, enlargening the size and creating problems for the Yoshis. Luckily, with the help of the Mushroom Kingdom, the Yoshis were able to escape and rebuild their homes. The Mushroom kingdom invited people from all over the world to a feast for helping with the construction. That day Bowser made his first appearance in years.
"I would like to have everyone's attention." he started off by saying. Mario and Luigi jumped to attention at their invader. "I am announcing my retirement. I no longer feel it necessary to fight with you because there is nothing I can do to win! So after years of thinking it over I've decided. You will never see my face again." There were many whispers as Bowser left them, but soon everyone was rejoicing.
When all the yoshis returned to their homes Kuroyoshi, the protector of the Super Happy Tree was in distress. He explained that while everyone was away he was attacked by an unknown foe who poured a strange potion on the roots of their beloved tree. All the Yoshis saw that their tree was experiencing strange side effects. The tree grew double in size and formed two halves. One half shined a bright radiance with healthy empowering fruit, and the other half cast a dark shadow of it's former self with dark, yet ripe fruit.
Suddenly the happy peaceful Yoshis began having new strange emotions, powerful emotions they never felt before.
They became angry with each other or sad when someone was angry with them.
Some Yoshis started to change in their appearance...
Others began to change in other ways...
By becoming bad.