The Changeling
In both Warhammer Fantasy Battles and Warhammer 40,000 the Changeling is one of Tzeentch's servants and is a total dick. He's special type of Horror that has the ability to disguise itself as anything, and uses that ability to wreak all manner of mischief. However because it has shifted forms so many times it forgot its original form. Only Tzeentch knows what the Changeling originally looks like, and commonly uses this as a bargaining tool to bind its loyalty to him.
One of the most famous hijinks involving the Changeling happened when it impersonated the Governor of the Hive World Raxos and threw it into total civil war, eventually conjuring a warp portal and having its other daemon buddies join in the fun. As usual, the Grey Knights were called in to contain the situation, which resulted in the total pacification of daemons in the uprising. However, the Grey Knights could not account for the Daemon that started the war and assumed that it escaped in one of the evacuation shuttles that were taking refugees off-planet. So in typical Inquisitorial fashion the Grey Knights shot down all of the fleeing shuttles, killing thousands of civilians in an effort to kill off this single daemon that MIGHT have boarded one of the shuttles. Overall, whether the Changeling was there or not, Tzeentch was most likely pleased by the amount of "Just as Planned" it did to bloody an entire planet.
Another one of its famous pranks was when a renegade governor's fortress was under siege by the the Dark Angels, he summoned up a Keeper of Secrets for help. You can probably see where this is going. When the governor asked for something "to end the siege", the "Keeper of Secrets" only gave him a mysterious device and fucked off. When he activated it: Surprise! The Deathwing popped in and gunned him down; it was really a homing beacon stolen off of a wrecked Ravenwing bike. The lesson is that you have to be really specific when you make a bargain with daemons, and that Tzeentch isn't above helping out the Imperium to troll his Chaos BBF Slaanesh.
The Changeling's dickery isn't limited to mortals either. In the new Daemons codex, he cut Slannesh's hair off and Whoopy cushioned Khorne with nurglings. Seriously! It also attempted to thwart Ahriman's plans of undoing the Rubric by impersonating huge numbers of people, coming dangerously close to succeeding before Tzeentch changed his mind because another hapless minion had done it for him. And in the new Curse of the Wulfen campaign book, it impersonated Azrael's of the Dark Angels to order a bombing on the Space Wolves' home world of Fenris (If this reminds you vaguely of Prospero, it should). Not only that, but it impersonated Azrael, and released a prisoner from the Rock. (Because the DA aren't paranoid to have any wards against imposters...)
