"What can we really comprehend? Does comprehension even exist, Hmm?.... Is this really happening? Yes, no, maybe... One thing I am sure. The only thing I can believe without a shadow of a doubt, is... THAT YOU SMELL LIKE POOP! HOHOHYORGHYORGHHYORGO!"
While Tzeentch had existed for millenia, most the time he schemes in the background and acts through his Lords of Change. One noteworthy occaision where he acted directly, however, was in engineering the downfall of the primarch Magnus the Red. Being a powerful psyker and always peering into the Warp, he became an easy target. Tzeentch began by infecting the Thousand Sons with the "flesh change," a mutation that would turn them into abominations. Magnus sacrificed his own eye to seek a cure for this change, to the point that his eye never existed. While it succeeded, Magnus was now drawn to the power of the Warp.
Tzeentch told Magnus of Horus' imminent betrayal of the Imperium of Man. Desperate to warn his father, Magnus attempted to send a psychic message directly to Terra. However, he ended up destroying the Webway of Mankind and created a portal through which daemons could invade Terra. To ensure Magnus had no choice but to side with Chaos, Horus ordered Leman Russ to attack Magnus. Magnus and his legion survived, but now had to side with Horus, and the flesh change returned with a vengeance.
Magnus' soul was now bound to Tzeentch's service, against his wishes. When Ahriman tried to cure the flesh change by using the Rubric of Ahriman, most of the legion turned into undead automatons. Magnus was furious, but could do nothing as Tzeentch approved of this move.
While certainly a horrifying abomination, Tzeentch is revealed to be little more than a schoolyard bully who annoys other people with his overthinking. He'll frequently claim that everything that's transpired is according to his desires, but Magnus revealed that he can't actually see the future and just relies on Kairos Fateweaver - that is, if he's not feeling too lazy to consult him. Tzeentch will pull various pranks just to annoy other people, such as sending Pink Horrors disguised as pizzas into Nurgle's plague garden.
In flashbacks, it was revealed that Tzeentch used petty insults to bully Magnus into breaking past the Webway barrier, comparing him to Leman Russ to anger him even more. Once Tzeentch acquired Magnus' soul, he treated him like an abused stepchild and would threaten to lock him in his room without supper. Tzeentch was forced to give up the soul when the Emperor psychically manifested himself to challenge Tzeentch by poking him repeatedly in the face with his sword.
Bitter at the loss, Tzeentch claimed that the Emperor had no legal right to Magnus' soul and attempted to steal him back during his trip to Nocturne. It was only after Kitten challenged Tzeentch to a Children's Card Game that Tzeentch was forced to back off, having been summarily defeated once Kitten saw through his cheating ways. Despite using a deck full of banned cards and mind-reading his opponents, Magnus was able to distract Tzeentch long enough for Kitten to outwit him.
- "What are rules really, though?"
- "AN INTERESTING QUESTION, ASSHOLE!"
- "So what are you going to do about it, big guy? You gonna tell on me to dad-athema and have your Administratum send me gold spangled glitterbombs? 'Cause I'll let you know right now I change addresses every five minutes." 
- "JUST AS PLANNED!" 
- "I can get you a cheap haircut and a cool new financial adviser if you give your soul to the empyrean!"
- "The throne is falling! Enjoy your economic instability!"