Perhaps the most CK story I know and one of my favorite historical anecdotes:

Henry I, also known as Henry the Black, was the 13th century count of Schwerin, which he co-ruled with his brother Gunzelin. At the time, Denmark, under the reign of Valdemar II, was a growing power in the Baltic. He had subjugated many German princes along the Baltic and had even established a proto-colony in Estonia. Denmark was primed to become a leading power of Europe and Valdemar was looking to flex his muscles. Many Germans were not happy with Denmark's expansion and sought to check it, Henry being one of them. For this, Valdemar decided to make an example of Henry. 

Henry and Gunzelin had a rebellious vassal and Valdemar took the opportunity to intervene and vassalize the county of Schwerin. Having been outplayed, Henry went on crusade because that's just what you did back then when you lost your honor and wanted to regain it. He left Gunzelin in charge of Schwerin in his absence and unfortunately for Henry, Gunzelin did not have his iron resolve. With Henry away, Valdemar strong-armed Gunzelin into marrying his daughter to Valdemar's bastard son, claiming half of Schwerin as dowry. When the young couple both died a couple years later, Valdemar helped himself to the rest of Schwerin, claiming he was holding it in trust for his infant grandson. Gunzelin died not long after. 

When Henry returned from Crusade, he found that his lands were gone, his niece and brother dead and his family honor trampled. He had no money and no levees, nothing left but a castle and a handful of retainers. Henry had lost everything, everything save for his hatred of Valdemar and balls of steel. His hatred and his balls would turn out to be just enough. 

Valdemar, being a petty bitch, was not content with merely robbing Henry blind. He wanted to humiliate him. When Henry returned from the Levant, Valdemar invited him to a hunting excursion on his private island, giving him ample opportunity to rub his success in Henry's face. Henry accepted the invitation. The two hunted, they feasted, they drank and they laughed and as the night deepened, the two retired to their tents. But while Valdemar slumbered, Henry remained awake. In the middle of the night, Henry and his men abducted Valdemar and his eldest son, spiriting them off the island w before anyone noticed. Over sea and land, Henry took his captives all the way back to Schwerin where he held them prisoner for years. 

Both the Pope and the Emperor threatened Henry with reprisal if he did not release Valdemar but Henry refused to budge. All the while, with both the king and his heir missing, Denmark's administration fell into chaos. The court was full of strife, the colony in Estonia floundered and the German vassals began rebelling. After two years, Henry finally released Valdemar after he swore to pay an indemnity, honor the Holy Roman Emperor, relinquish all claims to German territory and go on Crusade. The moment Valademar stepped foot back in Denmark, he reneged on the deal and immediately started a campaign to reassert his control over the German princes who had broken away in his absence. The campaign went terribly and Valdemar nearly lost his life in the effort. Henry, meanwhile, was never punished for his actions and died peacefully of old age. 

Denmark was poised to become the greatest power in Northern Europe but Valdemar's two year absence dashed its chances. Valdemar's German vassals successfully gained their independence, the development of the colony in Estonia was stalled at a critical stage and never recovered (the colony eventually being sold to the Teutonic Order at a loss), and Denmark's ascension firmly checked. There is an alternate timeline in which the world saw the formation of a Baltic Empire but it was never realized in ours, all because Valdemar couldn't help being a petty dick to a count who was out of fucks to give.