Fuck I had a spicy run in. My friend lost Lithuania to my heir, but my heir was going to unify my Kingdom with the Byzantine Empire. My friend assassinates my heir and I discover this, I'm at this point immediately livid as he just cost me the Byzantine Empire. He wanted Lithuania, but I meme'd him saying I wouldn't - in reality I would have given him Lithuania. He says that he's going to keep assassinating my heirs until he gets Lithuania, at this point I'm telling him that warning works before you start assassinating heirs, and is rather alike the Americans warning the Russians after they've already nuked the Russians. He continues to try assassinating my next heir, at which point I remind him that I have four more heirs with claims to every single one of his Kingdoms, and send 70,000 troops to take his land. When the troops arrive he is shocked I actually sent them via truce break, I say of course - you're killing my sons. He calls in our third player in the British Empire, and before long it's looking like the first world war.
After some intense diplomacy, we manage to negotiate a diplomatic solution, wherein I would leave Lithuania provided it was put under the protection of a Greek ruler, to protect all the Baltic Greeks. He would pay me 2,000 gold in return for the loss of my heir, while he would help me regain the Byzantine Empire. In actuality he married the Byzantine Empress and her heir to his heir the moment I gave him peace and withdrew my troops; when I delivered him the ultimatum warning him why I shouldn't destroy everything he has even at the cost of my Empire, he sent me a private message detailing how he had made the marriages to thwart the British Empire's own attempts to secure the Byzantines, and that our prior anti-British pact remained in force. He broke the marriage and helped me remarry, but Byzantine politics fucked all of that up. It's now a sheer mess, but he is correct - when we port into EU4, Britain will be too powerful, and efforts must be made to ally Greek and German to thwart Anglo