The end of the Psychic Wars and a worldwide economic depression lead directly to the rise of two opposing forces, who's deeds have shaped the world for nearly the last hundred years. The Thieves' Guild and the Assassins' Guild. Under Colun III they gained power. They controlled things behind the scenes, and at a subtle underground war with each other.
This was not enough for the Assassin's Guild. Seventy years ago, they decided to take out the young Colun IV after he assumed the throne. They sent Colm, the Assassin.
Colm himself is a mystery, books have been written about him. I have met him, and passed muster. The one fact that was unassailable is that he had the Black Blood. No one knows who created the Black Blood during the Psychic War. Accusations will never stop. Personally, I think that it was a lone madman, acting without sanction. It was powerful in Colm. It allowed him to blend with shadows, and ignore Arcane and Divine spells. Later in life it even protected him from Psionics as well.
With all of that power, he was unstoppable. He snuck into the Imperial Keep, and poisoned the drinking water. With everyone asleep, he snuck into Colun IV's bedchamber, and struck downward with his blade from The Land Unspoken Of. If it hadn't been for the layers of spells and magic items that he had, along with the timely intervention of his personal Cult Guardian, he would have died.
As it was, he lost an eye and part of his jaw. Colm barely escaped with his life. The Assassin still had a very successful career after that. He aided the Thieves and eventually took over the Assassins'. Colun IV became a recluse after that though. The life he would have had was gone, he was a broken and scarred man. He was afraid to move too strongly against anything. His grandfather would have hung his head in shame. The failed attempt still succeeded in its goal of strengthening the Guild power. This continued for a decade, until the strength of the Thieves' Guild was finally broken.