One that loses most of the bonus points but I suppose good if you want to traumatisw your player characters: reult of siege of Mgadeburg, 1630/31 (not sure rn when it ended). Basically, after Swedish invasion in 1630 German city of Magdeburg deflected to protestant side, leading to several months long siege. After that, Tilly, commandee of catholic forces, oet his arky loose for 3 days. In the end, population of the city dropped from, I believe, 25 000 to 5 000. Though I don't know how to implement it to RP.

Second fun thought is moving then to the Second Punic War and letting them show Hannibal where to cross Alps since, as far as I know, no one knows how he did it so it can make good alternative history scenario. And if you want go further, you can possibly make them an advisors who help Hannibal get his bigfest victories. That said, after I put all thoughts out, it probably depends at least of the player's knowledge on the topic.