My favorite story comes from my first successful Zarathustran campaign. I'd managed to reunite Persia and was debating what to do next. This was before Rajas of India, so I'd hit the eastern end of the map. To the southwest were the Abbasids, to the west were the Byzantines. I could take on either, but not both, and attacking one would be exposing myself to the other, so I pushed north instead, into miscellaneous Tengri territory. The king of Hungary still had his boatloads of event troops, so I had to strike and win my wars quickly; a protracted fight would give his troops time to arrive and help my enemies; I'd probably still win, but be drastically weakened, inviting an attack from the Abbasids or the Greeks. I bided my time, waiting and watching, until (surprise surprise!) the king of Hungary was assassinated (not by me) and his Catholic son took the throne, immediately marrying a Bulgarian princess. I begin stomping all over the Tengri, but I'm plagued by revolts and faction issues, and when the dust settles twenty six years later, I'm bordering all three of the area's other superpowers; I've reached the Hungarian-Bulgarian border; the king and queen are married to each other, so it's functionally almost one state. And here I pause to reassess and rebuild my forces.
While I'm doing so, the queen of Bulgaria declares war on me. In the time it takes me to maneuver my forces, she's managed to take one province- the one I accidentally awarded to my firstborn son and heir. He's captured and, oddly for the AI, executed. It's okay; I have a second son, a backup heir: not as good, but he'll do. I vow revenge, but decide that the best course of action is to slaughter as many troops of hers as I can, to make my subsequent attack easier. I do so, but the very day after we sign the treaty, my second son (and current heir) dies from poisoned wine. The culprit? The queen of Bulgaria. Now I'm MAD. I have no other sons, and my succession laws are agnatic. I'm in my sixties; my wife's fifty-eight, and Zarathustrans can't divorce. Even if I kill her, it's even odds whether I'll get another son in time; it looks like it's game over for me. But I'm not going to let this piddling AI get away with murdering my sons. I declare war and send in all my forces, hoping for a quick victory before any other Catholic nations join in. The Hungarian-Bulgarian armies gather for a last desperate stand at their capital, but they don't have a chance; every last soldier is slaughtered- and so, I note amusingly, are the king of Hungary and his son; severing the alliance with Bulgaria. "Ha!" I think, "Now you know how it feels!" I take the province, and am surprised to note the warscore immediately shoots to 100%- until I see that, among several others, I have captured the queen! Since I've essentially already lost the game, I hover my finger above the execution button...
And then I remember: I'm a Zarathustran. And Zarathustrans... can take concubines. And if they're your prisoner, they can't say no. Cackling maniacally, I concubine (or whatever the verb is) her, realizing that if we have any sons, they may well inherit Bulgaria. Then I noticed that in the few days between the death of her husband and her capture at my hands, she remarried. I check her new husband out, and note that (among other things) he has a -50 opinion of me for "spouse snatcher". Not only can you do it, it has a specific named penalty.
I like how evil this game lets you be. I love how evil it encourages you to be.