Okay, here goes. *deep breath*
It all started 30,000 years into the distant future, when some big douche with fabulous hair and golden armor the Emperor of Mankind created his 20 superhuman demigod sons, known as Primarchs, in a lab to help with his conquest of the galaxy. Each of the primarchs got an army of superhumans, known as Space Marines, born from their own DNA gene seed. One of these sons were Magnus the Red, and his army was known as the Thousand Sons. At birth, four assholes called the Chaos Gods yeeted the baby primarchs into different planets, and Magnus landed on one called Prospero. Thus, the Emperor had to use the limited gene seeds he had to create his Space Marine grandsons and help each army find its daddy. The Thousand Sons got their name because there were only 1000 Space Marines successfully created from their gene seed before it expired and they had to find their father, Magnus. During his time in Prospero, Magnus grew up with a not-so imaginary friend called Tzeentch who stole one of his eyes in exchange for knowledge and power. Once Magnus was found, the Thousand Sons could finally be more than just one thousand, but they still kept the name because they thought it sounded cool. One day, when Magnus was busy studying abroad, Tzeentch gave him a little heads up that one of his brothers, Horus, would betray the Emperor and declare a galazy-wide civil war that would wipe out most of humanity. However, Magnus was stuck behind an intergalactic border wall that his father built. While he was initially hesitant, Magnus eventually broke through, with a little coaxing from Tzeentch. This pissed off the Emperor, and he decided to put Magnus in time out. Then, Horus came to Magnus and convinced him that maybe the Emperor is kind of a deadbeat parent, and that he should join him. When things were going bad, Magnus went back to Prospero to sulk, but was chased down by another one of his brothers, Leman Russ, and his army, the Space Wolves. Magnus and the Thousand Sons got their asses kicked, Prospero was destroyed, and the only reason they were saved was because Tzeentch decided to take Magnus's soul as his own property, turning the once loyal primarch into a Daemon Prince. In the process, many of the Thousand Sons turned into literal dust within their armors, thanks to Ahzek Ahriman (the mage with horns up there) trying to save them from turning into horrible eldritch monsters called Chaos Spawn while under Tzeentch's command. This process was known as the Rubric of Ahriman. Today, Magnus and the Thousand Sons are among the armies of chaos who use the most amount of psychic powers, all thanks to Tzeentch encouraging it. Even before he fell to Chaos, though, Magnus was still a very powerful sorcerer, one of the strongest in the whole galaxy, actually. Second only to his father, the Emperor of Mankind, who by the way killed Horus by deleting his soul from existence, but not before Horus turned daddy dearest into a quadriplegic that now spends the rest of his life rotting away in a golden generator chair.
Yes. There are also a bunch of books and a few cool fan made series made for it, especially If The Emperor Had A Text To Speech Device, or TTS for short. Although, that one is basically a comedic parody.