On one of his many journeys throughout the multiverse, Garruk was out hunting a rare beast on some distant plane covered in jungle, a hunter's paradise. As Garruk tracked the beast farther and farther into the thick rain forest he began to feel an overwhelming source of green mana originating from the trees around him. Soon the feeling was too much and Garruk stopped to investigate the power resonating form the trees. That is when he realized what made this plane so special. Each tree had its very own mana source and they all seemed to be feeding into one central location near the center of the plane. Garruk decided to venture there to investigate it further, the hunt would have to wait.
Upon arrival at the center of the jungle-covered plane, the planeswalker discovered the source of all the mana: an elemental of epic proportions. This elemental seemed to be drawing in all the excess mana produced by the trees and feeding off it. Garruk gazed in awe as the giant took in more and more mana.
"What immense power! Could I be the first to witness such a great creature with such awesome power?" Garruk thought as he stared at the elemental. It was at that moment that Garruk decided he would defeat the beast in battle and make it one of his loyal pets or die trying.
An immense battle took place, and after much resistance, the mighty beast yielded to the powerful planeswalker. At that moment the creature formed a mental link with Garruk and told him this: "It is obvious you have talent, and you show the qualities of a true warrior. From this moment on, I, Omnath, the Locus of Mana, vow to follow your command."
And this is the story of how Garruk became ruler of the jungle plane known as Sylvania. He also has Emrakul over for tea on Thursdays.