When the Primarchs were spread throughout the galaxy by the forces of Chaos, one came to land in the far north west of the galaxy on a remote ice world named Fenris. He was adopted by a Fenrisian she-wolf and raised among the wolves, with his two wolf brothers Freki and Geri.
One harsh winter, the wolves attacked a village and stormed the food storage facilities. The villagers fought off the wolves, but Leman Russ fought them off so viciously that all of his brothers escaped alive. Thengir, King of the people of Russ, sent a raiding party of hunters and villagers to remove this menace for good with poisoned arrows and knives sharp enough to slice through oak.
The raiding party killed many members of the pack, including the she-wolf and one of his wolf brothers. Leman Russ was shot many times, and then captured, and brought, bound and gagged before King Thengir. Leman Russ proved himself to be more than just another wolf, and was eventually taken in by the king himself.
Among men for the first time, Leman quickly learned their skills, showing a natural aptitude for the way of the warrior. He learned to speak, and mastered their weapons - iron axes and swords. He was quick to roar with laughter or sing tunelessly and he slowly realized that he was more human than wolf, but superior to both. Eventually, Leman Russ became sufficiently civilized to warrant a true name. The King named him Leman (Leman of the Russ).
Later in his life, he is said to have been able to turn back whole armies of the king's enemies by himself without a scratch, to tear whole oak trees from the ground and snap them over his back, and to wrestle Fenrisian Mammoths to the ground. When King Thengir died, there was no question as to who should succeed him. Therefore, King Leman Russ took to the throne.
He was said to have been the best leader, no one could stand against him and it was not long before the tales came to the Emperor's notice. The Emperor recognized this figure as a Primarch and travelled to Fenris. He entered Russ's court, cloaked in runes of disguise and confusion. The natives shrunk from this new presence. Russ refused to pay him homage as the Master of Mankind.
Russ challenged the Emperor to a series of tests. The first challenge was an eating one. This the Emperor lost.
The second challenge was a drinking one. This the Emperor also failed. After failing these tests the Emperor called Russ nothing but a glutton and a drunk which led Russ to the third challenge.
For the third challenge Russ boasted he could defeat the Emperor in combat. This time, the Emperor defeated Russ, felling him with a blow from his power glove. Leman admitted defeat and swore to serve the Emperor.
Just weeks later, Russ was placed at the head of his Legion (newly named the Space Wolves) and joined the Great Crusade. He was armed with a thrice blessed suit of armour and a new sword forged from the maw of the Great Kraken Gormenjarl and reputably, the blade could cleave the ice mountains of Fenris in half.
Many years passed, and many more battles. Thousands of worlds were reclaimed by the Imperium, but eventually the golden age came to an end. The Horus Heresy and Leman Russ were intertwined to begin with, but they were absent from the final battles. The Emperor sent Leman Russ and his Space Wolves to deal with Magnus the Red and the Thousand Sons legion, who the Emperor believed to be the true heretics; Magnus had used sorcery to warn the Emperor of Horus after being told not to. Prospero was assaulted and bombarded. Thousands died within days. In the final battle between Russ and Magnus, it is said that Russ blinded Magnus and then broke the Cyclops's back over his knee, and just as Russ was about to deliver the final blow, Magnus pledged himself to Tzeentch and was ascended to Daemonhood.
Years passed after the Horus Heresy. Russ, at a great feast with his Legion in attendance, stood and said: "In the end I will return. For the final battle. For the Wolf Time." He then left the Fang, along with his personal retinue, and embarked into the Eye of Terror. The tale of his disappearance is retold every thousand years by Bjorn the Fell-Handed, the oldest dreadnought still in service. He is said to have served with Russ ten thousand years before. Every once in a while, the Space Wolves embark on what is called the Great Hunt to search the galaxy for their missing leader. Though they have searched far and wide, they still haven't found any sign of their Primarch.