The deck of many things drove an entire campaign for me once. When the game began to drag, some bored players started drawing from the deck. Things started off well, with them getting gems and bonus experience. One of the PCs got her own keep, which slightly rewrote the history of my campaign setting. That same player drew another card, but nothing happened. She thought nothing of it, and the group continued on with the game.
She had drawn the "your closest friend betrays you" card. Her lover of 15 years suddenly usurped her power and took the keep for herself. The campaign became a quest to serve vengeance to the traitor and win back the miraculously gained land. The game culminated with the group breaking into the castle while guards surrounded the area. The PC who got betrayed cut off her ex-lover's head and, in an act of defiance, tossed it out the window at the guards. The guards began storming the castle. In desperation, the group turned back to the deck of many things. (The deck had proven so interesting that I ignored the rule that prevented characters from drawing from it multiple times.)
This time, the PCs didn't have the same luck. One had to fight death. Another lost levels. One lost all of his equipment and was then imprisoned. Another earned the emnity of a powerful outsider, which would later kill him. Finally, one of the players got three wishes. His first wish got rid of the guards outside the castle and saved the day. Then, much to the anger of the player who had been imprisoned, his second wish was for immortality. That wish transformed him into an undead monster, so his final wish was to undo said immortality.
The PC who owned the deck was subsequently killed, and it wasn't until several decades later (campaign time) that the deck was recovered. It now lies buried in a place that only two characters know about. They have vowed never to use it.